I know I'm coming to this a bit late but here is my 2p.
First of all, I think the idea of having the inserts adjustable is an excellent one. I can see how that would give you very high levels of accuracy and precision. Excellent. I also like the fact that one board can be made to do several jobs - it saves on storage space.
Personally I would want a board longer than 300mm. If I am shooting with my No7 I don't think that 300mm would be a comfortable length, especially if the workpiece was wide. I've used cutting board material for other taks, like zero-insert plates, and as you say, it's not easy to find large cutting boards. But AX and Tilgear sell UHMW material which would, I think, be perfect for this sort of application. My own shooting boards are faced with Formica, which also works well to reduce friction, and of course, size is not a problem.
My bench is up against a wall, and the bench itself is not that deep, so sometimes I have a problem if I am using a long plane pushing away from me - it's easy to hit the wall. I intend to make my next one skewed at 45 degrees, so that I have more space ahead of me. It also means that very long boards will need to be supported on a a stand, but at least they won't be fouling my SCMS, which is what happens at the mo. I need more space!
Mine also has an adjustable sacrificial fence, which reduces breakout. I nudge it out then plane it away until the plane stops cutting. At this point the lateral margin of the plane is rubbing on the shoulder of the runway. When the workpiece is mounted now it will be supported right up to the end of the cut.
Finally, I keep a separate blade for shooting. It has a square grind, no camber at all, which gives me a perfectly flat end to the board.
I have an article on shooting boards that I started yonks ago and never finished. I thinks it's probably time to revisit it, especially as I need a new one myself.