You could narrow the back bracket of an inexpensive slim mount hdtv bracket, and hang the hdtv that way. Any frame could use some sort of alignment posts, whether that is created by wood dowels,
or a home built French cleat. Maybe a couple of dabs of Velcro to keep the frame pulled in close to the wall at the bottom.
I am doing a flat surround, and the plans are to is use simply use some acorn nuts to finish off the four mounting posts. I want the frame flush with the wall, so I will recess a shallow wood box, and simply 'float" the frame
even with the drywall. I will keep a reveal around the frame so ventilation is accommodated. I doubt some acorn nuts would look attractive with a wood frame, but I am looking for a more industrial look.
I also have seen a rotated hdtv and it's covering frame hosted by two little L brackets up top, where the eye doesn't see them. Maybe that could be as simple as two flat brackets, with the bracket routed into the frame, so
everything sits flush.
I expect there are many ways to engineer a frame mount, up to some mounting posts and springs.