This is primarily to say that I don’t see the use at issue as having come from the moving 1756 use, and but for its length, this “answer’ would have been posted where it belongs, as a comment:

Not only do I not see that 1756 use as a possible source of the “C’est dommage” sense, but I even have issues with Capt. Bossu’s interpretation of it (in fact, I even question why Bossu felt any need at all to tell his readers what “de valeur” meant to the brave father).

Mainly because of the “Mais” that begins the 2d sentence, I interpret those words using their literal, not-needing-explanation sense of “bravoure/courage/vaillance”:

“My son is facing his death with courage, BUT (please spare him for the following reasons.)”

If the father had been “merely” expressing how

“fort/extraordinaire”

his son’s death is/would be, then I think someone with such a good command of French would have perhaps introduced the reasons for the death being “fort/extraordinaire,” not with “Mais,” but rather with “Parce que/Car/Puisque”:

“That my son faces death is extraordinary/an exaggeration BECAUSE (of the
following reasons….)”

In addition to the use of “mais,” I’m tempted to question the good Captain’s “fort/extraordinaire” interpretation also because I don’t think it’s impossible that he, and his intended audience, with their arguably less spiritual, “western” understanding of “bravery/courage/valor” might not have been able to fully grasp what those concepts meant & how important they were to the Collapissa/Acolapissa People.

Another possible interpretation of “C’est de valeur” that would, in my opinion, also jive better with “MAIS” than Bossu’s intrepretation could be

“It’s justifiable/understandable/right that my son must die, BUT
[please spare him…”,

but I still opt for the “with courage” interpretation, which would have required no clarifying footnote, in my opinion.

To make this more than just a statement of the issues that I have with Captain Bossu’s interpretation, I will add that agree with the last two comments on this Wordreference thread where the possibility of a fear of mentioning the word “malheur” (born of the old adage «Ne parle pas de malheur ») might be behind using “de valeur” instead.

(a possible connection with the negative notion of “dévalorisation/C’est dévalorisant” also crossed my mind, but I like the “malheur”=”de valeur” explanation much more)

After having now gone through the material, here are my thoughts. To preface this, I’m only proposing the various means by which this semantic shift could have taken place, not deciding between them. Thus, I’m only answering the procédé part of your question.

The two basic paths I’m laying out are either (1) that de valeur either had or came to have the sense of fort, extraordinaire, which then swung over to the negative side instead of the positive thanks to the inherent neutrality of that sense, or else (2) that it did have a positive meaning, but irony flipped its valence, as you suggested. Clearly there is some overlap between these two stories.

Bossu’s gloss and the Muskogean original

First, I think Papa Poule is on the money with his second interpretation of de valeur as it occurs in Bossu’s text. Bossu’s gloss for the word makes little sense in context (though it might have if the opening of the "harangue" weren’t immediately followed by "mais").

Since the idea of valeur is tied to the verb valoir and can mean ce que vaut une chose (TLFi étymologie), it makes good sense to me that the father is saying, "It is indeed fitting/warranted that my son should die, but being young and vigorous, he is more capable [than I] of providing for his mother, his wife, and his four young children."

However, the scenario is complicated by the ambiguity of Bossu’s footnote:

Le terme de valeur est un mot qui signifie, en leur langue, ce qui est fort ou extraordinare.

The linguistic layers must be disentangled. Presumably by "in their language" he means one of two things: a patois or dialect of French, which I doubt; or the Acolapissas’ own Muskogean language, in which case our only access to the Muskogean term is by the intermediary of Bossu’s rendering de valeur. However, the point of Bossu’s footnote is to say that the literal meaning of de valeur does not represent the meaning of the Muskogean, giving us little clarity indeed about the latter.

It seems that Bossu considers that the Muskogean in some sense (etymologically?) equates to de valeur at the time of his writing, yet actually conveys the meaning "ce qui est fort ou extraordinaire". That is, his observation is about of the same order as the French learner who reports that aujourd’hui "literally" means "on the day of this", but what it really means is "today".

This sort of astonishment is very typical of an early learner navigating a second language and realizing that many simple words can be broken down by an analytical eye into a curious "literal" meaning. (Indeed, it’s an essential strategy for identifying the meanings of new words.) That Bossu was often struck by words of this kind is suggested by the avertissement :

…d’autres [notes] pour expliquer certaines phrases emblématiques dont se servent les Sauvages dans leurs harangues.

By this I mean that just as a native French speaker uses aujourd’hui simply to mean "today" without thinking of the components that stand out to a language learner, for all we know the Muskogean word may have resembled de valeur in form only. If that’s true, de valeur was no more the best translation than "on the day of this" would be for aujourd’hui — though it may be of etymological interest.

Proposed intermediate stage: “fort, extraordinaire”

What is the significance of the Muskogean for our etymology? It contributes the first of the several hypothetical ways that de valeur could have come to mean fort, extraordinare.

1. Perhaps settlers were just as struck as Bossu was by the "literal" meanings of those phrases emblématiques, and allowed these meanings to creep into their language. This phenomenon is referred to as semantic borrowing. Bossu demonstrated it very well: he used the French phrase in what he felt was an unnatural way that must be explained to French readers, with the intent of mimicking the phrase’s semantic range in Muskogean. Incidentally, this effect has a long history: as Moises Silva notes in a 1975 article on semantic borrowing in the New Testament, the Septuagint Greek that translated the Hebrew Tanakh used Greek terms as though they were the Hebrew equivalents: for example, they used one Greek word for "lake" and "sea" as Hebrew does, rather than distinguishing the two as Greek did. No new word need be borrowed for the semantic range to spread. Hence, it would not be surprising if by long cultural association, Canadian French speakers tended to borrow not only the forms but sometimes the semantic ranges of Indigenous words.

2. Perhaps, on the other hand, Bossu’s account and others like it could have influenced de valeur. That is, the semantic range might have shifted in Canadian French without direct reference to the Muskogean, only mediated.

3. Perhaps in Canadian French (or that of the original settlers’ home regions when they left France), de valeur still had a neutral sense similar to one of the meanings of its root, Latin valeō : with "être significatif, puissant". Compare even the modern senses noted in TLFi entry IV – D "Importance, portée d’une chose" or I – B mettre en valeur, one of whose senses is "mettre en évidence, en relief". On this view, the Muskogean term is entirely irrelevant, since de valeur would not have gained but retained this meaning.

Between the above, I consider (1) more likely than (2), but between (1) and (3) I would be undecided.

However, I reflect at this point that after consideration of the TLFi entry and of valeō, as well as similar terms in other languages, either way there is some semantic overlap between "to be worth, to signify" and "to be strong, to be noteworthy". Expressing something’s worth is not far removed from saying that it’s worthy, nor identifying its signification from saying that it’s significant. (Incidentally, this is another demonstration of the principle that the pragmatic value of an utterance gradually colours the denotations of the words used.)

In light of that, we can add the remark that Bossu hardly needed to provide his interpretation for de valeur at all — unless the meaning was firmly positive by that point and hence ill-fitting in the father’s remark. For example, it might have come primarily to mean "valuable" or even "excellent".

On the one hand, this is plausible given that the TLFi etymology of valeur lists mostly positive senses long before the 18th century. On the other hand, it would mean that the "literal" meaning of Muskogean term had such a positive connotation, which is harder to reconcile with its appearance at that juncture, though not impossible, if Bossu’s analysis is correct (but remember that his particular interpretation hurts the text’s coherence!).

In any case, I think we’re obliged to posit this fourth option:

4. There never was a fort, extraordinaire stage natural to Canadian French. It was only positive.

Deux réponses très détaillées et j’en ai malheureusement peu à ajouter. J’y vais quand même, ne serait-ce que pour faire démolir mon idée sur la chose afin qu’elle ne me tiraillât plus.

Nous avons ici un allié des Français, dont la langue maternelle n’est pourtant assurément point le français. Il dit aux Français que « c’est de valeur que [s]on fils meurt ». Cette utilisation de valeur pourrait-elle être due à un quiproquo passé par rapport à la signification exacte du mot ? Un fusil, par exemple, a une valeur par le fait qu’il coûte un certain montant, et aussi parce qu’il change le quotidien en facilitant la chasse. La qualité positive de valeur aurait-elle pu échapper à ce peuple du fait que la valeur en tant que montant de son achat constitue plutôt une perte de leur part, contrebalancée par un gain dans sa future utilisation ? De là, le concept de valeur aurait bien pu en effet être évalué comme une mesure de conséquence sur le monde, plus que comme une mesure du bien ou du mieux offert par la chose de valeur.

Puisque les conséquences de la mort d’un jeune homme et grand chasseur sont importantes, ils auraient très bien pu alors indiquer que cet événement avait une valeur, qu’il était de valeur. On pourrait imaginer aujourd’hui une description qui affirmerait simplement que c’est quelque chose, neutre quant à la qualité de l’évènement, mais clair quant à son importance.

Il me semble que l’explication offerte qui jette un pont entre « c’est de valeur » et « c’est d’importance » est la plus intéressante, mais en parlant d’extension plus ou moins naturelle, elle me semble mouler la réalité en un flot continu, en une métamorphose de proche en proche. De la même manière que l’économie ou la physique quantique procèdent par sauts sans états intermédiaires, je pourrais très bien accepter une cassure nette due à une mésinterprétation qui aurait fait boule de neige. La preuve qu’un quiproquo linguistique est possible serait l’analyse même que fait Bossu de cette expression dans le patois des Collapissas : la mort d’un fils est-elle forte ou extraordinaire ? Ne serait-elle pas plutôt de conséquence ?

Au final, cependant, je ne saurais même pas comment retracer l’existence de cette acception depuis ses origines jusqu’à nous. Je sais lancer des hyposthèses, mais les étudier et les confronter à la réalité, c’est une autre paire de manches.