Interesting, Jonny - I've been considering doing the same lately as well: move my MMGs to the back and get some 1.6QR's and replacing my current sub with a new sub (or two) up front. I haven't the foggiest idea what I'd do for the center channel, though. Have you sorted out what you'd like to do for the center channel?
I've been running my B&W 601s (or some such, don't remember the specific model right now) as my rears, and it's a bit of an odd combo - the Maggies aren't nearly as dynamic and in-your-face as the B&Ws. I bet as long as you have planars all around, it'd be an okay match, but...don't know for certain.
I do think if you want slam-in-your-face surround, planar's probably aren't the way to go. But for my tastes, and since my primary listening is 2-channel audio, I love the planar sound.
How would you wall-mount the MMGs? And why are the MMGs wall-mountable but the 10.1's aren't?