Using GAC at shows can be largely hit or miss. We have used it at FOTLK. (But due to other issues we can't do this show anymore.) VOTLK, we have not tried in years (that waiting room is killer). But we have been allowed to wait off to the side (outside) until the people start going into the actual theater. LMA is a nightmare. We have had mixed experiences. I don't remember ever being told a GAC cannot be used for shows. I just find the "assistance" offered is really what we need, so we can't do some shows.
When the GAC is used for shows, it's usually for hidden disabilities, such as someone with low vision, someone who can't do stairs, etc. I've never seen it work for a waiting area but then I've never asked for those accommodations either.