If brass wire that fine would be too fine, then the chain itself must be the finest that we have seen from any manufacturer, I think the finest chain that is usually available is around 40 links.I would guess it is 40 links/inch metal chain they've used and I hear what you're saying about just looping a piece of fine wire with a "hook" on one end to just drop through the hole in the piston arm. It is the job of forming the other end of that new piece (IE: get the wire bent around to a REALLY small loop where it would connect on to the original chain) that would be the killer.
Just reread your post saying the chain came off on the brake cylinder end, could you remove the D shackle from the piston rod, & make fit wire through there & into the chain, is the chain metal or plastic, if metal maybe solder hard wire to the chain & drop the wire through brake arm, if plastic do the same but use super glue.
I wouldn't like to solder or super glue it either although the latter would be the better option IMHO (because the chain is painted/treated it would not take solder readily I imagine).
In any case, like I've said before, the absence of the chain loses me not one wink of sleep....hahaha