Is this usage accepted in formal writing and/or understood in vernacular speech?

This usage is more common in written French and academic speech than in everyday’s speech.

If so, are there many other situations in which “chez [name]” may refer to something other than a location or dwelling of something other than a person or group of people? To what extent is such generalization of its usage deemed acceptable?

It is acceptable to refer to anything that can have a “house” (in a broad sense), i.e. everything that lives so not restricted to people and includes animals (e.g.: chez les baleines) and even plants (e.g.: chez les feuillus).

It might also be used when referring to objects that are personified like with:

Du cannibalisme chez les étoiles. (Science et Avenir)

The pharmaceuticals advice you referred to in your question Emploi de « chez » dans une précaution d'emploi is using chez not to mean “at someone’s home” but “with”.

Note: Chez l’humain is not very idiomatic, we’d rather say chez l’homme or chez l’être humain.

This usage is taken into account in the TLFi (chez, C) and it belongs both to the formal language and the everyday language.

C. [Avec des pluriels, des collectifs ou des singuliers de sens générique; le groupe prépositionnel désigne une classe d’êtres dont on décrit les traits ou les comportements spécifiques.]

I

There are several syntactic categories of noun phrases in which this preposition can be found. It can be translated in some cases approximately by "characteristic of" or "as pertains to", "for".

• Il n’y a pas chez l’éléphant d’instinct qui, comme on le croyait, le mènerait vers un cimetière quand vient la mort.
  (There does not exist an instinct characteristic of the elephant which, as we believed it, should lead the animal towards a graveyard when death approaches.)

• Chez l’enfant en bas âge vous ne devriez pas utiliser ce médicament.
  (As pertains to young children you shouldn’t use this drug.)

1/ [plural nouns] chez les enfants, chez les physiciens, chez les artistes, chez les étudiants, chez les allemands, …

Do not confuse "chez les allemands" as meaning "amongst German people" and "as pertains to the German people".

• Chez les allemands vous devez vous habituer à une sorte de politesse qui leur est propre. (Here, you can’t say "l’allemand" (not correct), whereas in the next sentence you can do that without changing the meaning.)

• Chez les allemands/l’allemand le sens de la discipline est plus fort.
  (As pertains to the Germans their sense of discipline is stronger.)

2/ [noun modified by a quantifier] chez l’immense majorité des professeurs, chez la plupart des hommes agés, chez certains des sujets, chez presque tous les malades, chez quelques uns des étrangers en vacance ici, chez l’ensemble des vertébrés, …

3/ [term referring to a group including pronouns and periphrastic terms] chez la plupart, chez le reste, chez eux, chez ceux que nous allons étudier, chez la plus grande partie,

4/ [generic, singular noun] chez l’embryon, chez la plante jeune, chez l’insecte, chez l’homme, chez l’enfant en bas âge, chez le cep américain, chez l’allemand,

5/ [noun or pronoun referring to a given person] chez Pierre, chez lui, chez ce monsieur,

particularly used for writers, philosophers, thinkers      chez Bergson, chez Sartre, chez Camus, chez Thomas Man,…

II

As the definition tells us this usage is specific to "beings" ("êtres") although there is much discussion as to whether things as embryos ("embryons"), vine stocks ("ceps") or plants are really beings. There is nevertheless a clear limit as indubitably inanimate entities can’t be used in combination with "chez".

• nouns that can’t be used

  • objects     maison, chaise, métal, papier, livre (chez le livre ancien, chez le livre moderne), tissu, vêtement, etc
  • abstract nouns     liberté, passion, feignantise, ardeur, fierté, abnégation, etc.

Addition due to a comment from user JD2000

Comments
I can’t produce such a specific rule myself, and it is very specific if we can’t link the species "book" to a genenral category. Nevertheless I am absolutely sure that I have never found such constructions for plain material objects or even not so plain, as for example preparations (chez la viande de bœuf, chez le fromage jeune, chez le travail de cet artiste, chez la peinture classique,…). Shouldn’t we keep things well separated so as to preserve a clearly defined usage? —LPH

Is the "chez" in those examples the same as the "chez" in @Destal ‘s example though? I notice that Destal’s example is in the plural, whereas yours are all in the singular. Could there be a difference in meaning analogous to chez les allemands = physically in their territory vs chez Balzac = in Balzac’s writing? In other words would ce style particulier qu’on retrouve chez le livre ancien be more objectionable than cette odeur particulière qu’on retrouve chez les livres anciens? — JD2000

Reply

Yes, the meaning is the same.
No, there is not the least difference between "chez + sing;" and "chez + pl.". Whether you say "chez l’enfant en bas âge" or "chez les enfants en bas âge" the same meaning is conveyed; there is one difference in the frequency of use: the plural form is sometimes preferred; sometimes also this preferrence is such as to make the plural forms almost unique (for instance "chez le mammifère" is hardly used).

chez l’enfant/les enfants.

chez l’adulte/les adultes

chez les humains/l’humain

chez l’être humain/les êtres humains

chez les mammifères/chez le mammifère

chez les insectes/l’insecte

The plain reason for the singular in my examples is that the nouns are mass nouns; the concept of "chez", not meaning specifically "physically" as you say and as if by oppposition to "abstractedly" but meaning "amongst" has nothing to do in this question. Nevertheless, you raise a subtle question, which is "What is the real difference in meaning between the two?".

The question as regards user Destal‘s example (chez le livre ancien/chez les livres anciens) is whether there is or not an established usage for this type of thing. An ngram shows that neither form i