Could either of the theaters absorb the queue?
Am I remembering correctly that there’s an exit door from Bourne AFTER the “security scan” but before the preshow room?
If so, you could use the Bourne queue plus set up a temporary queue plus the M&G inside the preshow room and have people exit out that door I mentioned already.
This setup would also allow people to see Art plus give the actor an entrance exit through the theater without letting guests into the actual theater itself.
You could also setup a queue inside the HMUS lobby spilling out into the area around the HMUS, use the theater as a temporary queue space, and have the M&G take place in in the theater itself near the exit (so guests don’t have to walk down to the stage) or in the exit lobby. The issue I see with that setup is entrance/exit for Art if you do the exit lobby.