Those hide-a-ways can be fun. I never did one out on the face of a wall or fireplace, but did several behind the chimney within a closet or, in one case, a laundry room. One was in stone. We left an opening in the back side of the chimney to insert a "deadman" beam to lift the lentil stone (top stone where the angle across the fireplace opening would have been) the stone had to be dragged onto the floor by means of a Jeep (A front door and back door were in alignment so heavy rope and rollers were used) and then placed on rollers and rolled to just in front of the fireplace. Then using block & tackle from stone to "deadman" beam, we lifted and slid the stone into place. Once the beam was removed, i cut a small stone to fill the opening in the backside of the masonry. You could see the joint, but it was hidden in a closet, so one had to know where to look. I never did one out in a spot where it had to be hidden by crafty joinery such as this one.
Tinker