The client also purchased a mount already, and while it's too big to fit in between the outcropping and the fireplace, we did come up with a weird plan. If he doesn't mind the TV being a tiny bit off-center on the fireplace, the backing plate can go around the outcropping. The arm that extends to hold the TV can then be fully-extended, moved up all the way next to the outcropping, and the TV is hung there. It would mean the TV would be off center by about one brick, but the way the furniture is laid out in the room it might not be awful.