I guess you have considered speaker grill fabric. They come in many different colors and their sole purpose is to be AT.
Are the wall going to be one piece or modules?
I haven't thought about it deeply but so far I'm thinking removable floor to ceiling panels. Something that would give the appearance of a fabric wall but easily moved so I can place the speakers behind and the screen in front. Something like that picture I linked too but less fancy.
Perhaps hinged..It looks as though Spandex would probably be a pretty good option considering the variety of colors/prints, price and acoustic transparency. I'm in the process of planning my living room and I'm incorporating the home theater so I'm investigating possibilities. I had an electrician friend over last night to talk about moving the ceiling fan and placing the projector where the fan currently is, running in ceiling/wall wiring and installing recessed lighting. Since it's also my living room I'm thinking of ways to downplay the theater aspect as well.
There is nothing either good or bad, but thinking makes it so.