We had a solid cedar fireplace mantle installed and it was interesting how they did it. They drilled holes into the brick with a masonry drill, then had rods that looked like rebar, and set them into the holes with epoxy gel. They propped them up so they'd be somewhat level. Once they set, they drilled holes into the back side of the mantle, coated the protruding pegs with glue, and set it in place.

This is a gotta-get-it-right-the-first-time kind of thing but might work for your situation.