yeah i agree with the 3d bit. if it were me, I would track out the mirror shot and put it as the reflection map on a material in maya. then you just map out the UVs on your polygonal mirror object and apply it. then you can do all the fun with cracking effects. render out the final shot and comp it underneith your character's fist with a corner pin to keep it in the right place.
sounds like a long process...only a couple hours though. good luck and dont stress too much over the shot because you can always reshoot it and have him really break the mirror hahaha.