While the safest way to install the mirror is with a ledge (l metal or bar) my client wanted a clean look so we went with the below solution.

I glued two large French cleats (plywood with a 45 degree angle) on the back of the mirror so it acted like you were gluing it to the wall, sheer wise.

Have your contractor make the cleats and install it. The only thing you really have to worry about is making the cleats too big on the sides where you will be able to see the wood, I held it back 3" from the edges and it turned out great. I hope this answered your question, let me know if you have any others.

Best,

Matt