The practical answer would be to have Mr. Harman or another maintenance person in the lift who would be able to provide assistance. As we've seen in the series, Mr. Harman is very aware of the current state of the lifts.

Mr. Rumbold would be tall enough that between the two of us we should be able to easily reach the ceiling if necessary.

I also wouldn't mind being with the set-designer and behind-the-scenes crew who knows where the secret entrance/exit is that allows actors to get into the lift without the audience knowing. I always figured one of the side panels (which are difficult to see on camera) opens very easily to allow actors to enter or leave the lift back-stage. In fact, you can't even tell if there ARE any side walls in the lift during some shots.

I've done several theatre plays, but we never had an elevator so I'm not sure how it would work. Often, you can see the elevator girl helping to pull open or close the lift doors.

I actually think Mavis from Grace and Favour would be a lot of fun to hang out with, especially if you had to wait in a lift for awhile.