Did you ever get your bracket figured out?
I was thinking gate hinge with the lag style not the bolt style. Drill a double tall hole in from the back and drill down through the top to hit your 'mortice' at the depth the hinge pin sits. You would slide the bracket onto the hinge lag and drop it onto the pin at full depth which would lock your bracket in against the wall without anything showing.
We do a Similar thing with a lag by cutting the head off and simply drilling a tight hole in the back then tapping the item to be mounted onto the shank. But that will not hold your bracket to the wall when the shelf applies leverage.