Have the actress playing Tommy's mother in an identical costume with one of the chorus, and do they would perform the scene as mirror opposites, with the Mother on the main stage, and her "mirror image" behind the empty frame.

 

Then, when it comes time to "smash" the mirror, they actresses simply run towards each other -- there is a SOUND EFFECT of crashing glass -- a light cue, perhaps a strobe or blackout -- maybe even a confetti gun filled with silver mylar pieces -- and the chorus dancer simply vanishes into the wings or behind a curtain -- suggesting a broken mirror instead of having to clean up shards of broken glass every night.