Although there is space free in the cast member car park, I think they wouldn't do this, as it would feel a bit odd to just walk behind the buildings on Main Street, and I just get the feeling that it wouldn't fit well.
The two other options are relacing the Town Square Theatre(it may be kinda popular still, but they could set up a meet and greet elsewhere, and would they really have two theatres on Main Street?) or Tomorrowland Terrace(which is barely used as it's out of the way, nobody really knows about it).