Hi mothball, the town hall in Maesteg (lovely as it is), probably isn't the right place for your search - you'd be better off contacting the council office, also on Talbot Street in Maesteg. It's a seperate building where the council meetings were/are held.
I know this post is a few years old, but I'm not sure if you'd gotten any further. It's been a long time since I've been in the offices, so I can't remember if there were any plaques inside.
Failing that, there is the miners library in Nantyfyllon, which is still in use, possibly as some sort of community centre. The miners library in Caerau burned down (more than once) around the 1980's - I lived opposite the shell of it before they eventually pulled it down (Library Road, Caerau).
I'm not 100%, but I don't recall a miners institute or library in either Maesteg or Garth, but I've never had the need to look for them either.