Could a 7 be easily turned on its side and used as a Center, or would it sound too different from the vertical sides to make that work?
Just drop off the sub, turn it in its side, and rotate the tweeter and your there. You'll have to build a trick looking stand for it, but it will work fine.
As soon as I can I plan to draw up the enclosure plans for a OB-5 turned on its side and one of the lower woofers moved to the other end. Since the lower woofers are only covering from 200Hz and down they can be moved around with no effect on performance.
Could the 7s be built so that the subs could go in a different location
I wouldn't move them around too much because they need to blend with the M-130 woofers. You can still add secondary subs with PR's (playing down lower) and place them where ever you want in your room.
The OB-5's play down low enough to reach external subs mounted elsewhere in the room. You could go with three of those across the front.
Is there a plate amp option that would have one or two bands of PEQ and adjustable phase?
I am looking into a plate amp option with variable phase. The ones that I used only have a 180 degree phase switch but in one direction (I don't remember which) it made them perfectly in phase with the M-130 woofers.
You can use any plate amp that you want on them really. It can even be mounted externally. Or it can be mounted on the inside of the speaker at the bottom of the enclosure just opposite of the outward firing sub.
These are some of the best things about kits. You can vary them slightly to make them just the way you want them to be.