I have been thinking of getting a muscle suit for a mannequin, basically just getting one of those DELUXE SPIDERMAN SUPERHERO ones with the legs and chest and arms, and just duct taping the living hell out of it to compress it to "normal" big size and not "holy crap the steroids are gonna explode" size.
I thought about it for a few minutes, and had an idea... What if you (or me, or whoever) used foamies on either the mannequin (glue) or a bodysuit on the mannequin to "build up" normal looking muscles. That way when the costume goes over it, it looks somewhat more realistic than GIGANTIC BICEP MAN that most of those costumes have.
A few layers of foamies and maybe some fiberfill, some epoxy or glue or duct tape or something to make it stick, and boom, you have a muscled out hero.
Crazy enough to work, or just crazy? Granted this is just for a static display piece, but I think it might be doable....