Hate to say this but that overhang you created might not be to code. Basically the heat or flame will get trapped under that overhang and could possible ignite. It might be close but usually the further the projection the higher the mantle or in your case overhang has to be.

If it is a gas fireplace it probably isn't a concern. If I was building a mantle there is no way to put corbels without disturbing the rock on the surround.

I would build a basic mantle that looks like a solid timber but the lower board would go against the rock and of course the upper piece would have a shallower depth. Looking at it it would look like a pieces solid say 8x8" piece of wood set into the wall.

You did a beautiful job. You don't need no stinking mantel. ;-)