I haven't used it, but MartinLogan has an optional swivel mount available for the Matinee speaker, giving it about 35 degrees of swivel. I'm not sure you need it, though. Unless your room is HUGE and your seats are twenty or thirty feet ahead of the speakers, your rears will be fine in almost any position that lets each speaker aim in the general direction of the back of each ear. Why not try them on a rough 4 o'clock and 8 o'clock positioning, then move them further back and see if you hear any significant difference. Balance with meter as needed.
The neat thing about human beings is we are serious predators, with the binocular eyes and stereo ears of a predator. Our brains are hard-wired to focus forward, which is why we can detect subtle differences like front width and height and depth. Sounds from behind the line of our shoulders are not given any where near the same attention. In my current room my rear speakers are actually on the side corners of the room, aimed in towards my seat which is only a handful of inches ahead of the wall. Not ideal, but in the real world that's the way it has to be. And it's pretty effective, because if your eyes can't see the sound source your brain automatically places it "behind" you.
