Action de s’agripper :

Je m’agrippe aux barreaux.

État qui dure :

Je suis agrippé aux barreaux.

De la même manière :

Je m’assoie sur une chaise (action).
Je suis assis sur une chaise (état).

La forme transitive agripper signifie plutôt attraper, accrocher :

J’agrippe cette personne, j’agrippe la lanière.

La forme pronominale indique qu’on est soi-même cible de cette action, et pour ces types de verbes, une forme de liaison plus forte :

J’accroche cette portière. Je m’accroche à cette portière. Je suis
accroché à cette portière.

J’attache cette corde. Je m’attache à cette corde. Je suis attaché à
cette corde.

Je suspends cette poutre. Je me suspends à cette poutre. Je suis
suspendu à cette poutre.

The pronominal form (la forme pronominale) makes sense to me.

I think that you are over-analysing this sentence and lending to Camus too much credit for multi-layered meaning.

One must also remember Meursault’s psychology throughout most of the book: He does not express emotions. So, trying to find emotions in his action of grabbing the bars does not fit well with the character or the spirit of the book, in my opinion.

The situation is far simpler, thus requiring a simple translation.

Meursault is in his prison cell, looking outside the window (with bars), looking at the sea (as stated), catching some sun (as stated) and fresh air (common sense), and possibly contemplating a world of freedom (left to the interpretation the reader). Since windows in prison cells tend to be rather high up, Meursault may have had to lift himself up by grabbing the bars and/or stand on a chair (thus needing to grab the bars) in order to look outside the window.

So, he wasn’t grabbed by anyone or “put in a state of grabbing the bars” (this translation would not work in French), but he was simply grabbing the bars as he probably had no other choice.

This action had been lasting and was still occurring when the guard came in.

So, the most literal translation of “que j’étais agrippé aux barreaux” would be “as I was grabbing the bars”; it is neutral enough to work in this context.

“as I was hanging on the bars” is a translation I read recently. It does not satisfy me because the meaning of the verb (to hold tightly) is too weak compared with the meaning of “agripper”.

I would prefer “as I was grabbing at the bars” to create a sense of tension, roughness or desperation. Clinging, clinching, clutching and grasping would provide a deep “dramatic” effect too, but I doubt that this was Camus’s first intention since Meursault does not express emotions and is distant.