Here's a quick sketchup of what I was thinking. Basically, I would have to build a backer box to just fit inside the joist cavity (12" oc, so I only have about 10 inches of width) to seal off the leaks. I'm not doing clips or channel, and will be doing a DD + GG directly to the bottom of the joists, to I'm only planning against sound leaks, not vibration. The bottom of the box would be flush to the edge of the drywall surface, so I can get clean corners.
Inside that backer box, I would have a little bit of room to angle the speaker slightly on two planes, both "front-back" and "side-side" which would let me point them a little bit toward the MLP.
I would probably only do the front Atmos pair this way, and either flush mount or just a little "side-side" tilt on the rear pair.
The grille would either have to be attached to the box edges, meaning it would protrude into the room (1/4" if I'm using 1/4" MDF) or I could install a lip inside the box to flush mount it. We'll see how things evolve. I hope that all makes sense.