I think we confused the poor guy.
Tai worked out a plan for the mantle/floating shelf by gluing together four 3/4"-thick pieces of plywood. He notched out the back of the shelf to acommodate a back support recycled from discarded 2"x4". He drilled the back support piece into the wall, slid the mantle onto the support, pounded in some finishing nails to connect the mantle to the support piece, then patched and sealed the mantle.
We've had a few people come by recently who said something like, "hey, you guys are finally finishing things up!" when the only difference between visits was the fireplace and mantle. It feels good to have the face on.
Here are some process photos: