I don't speak on behalf of Gunnar, but that sounds pretty pimp! From what I've seen, a full knobbie 650b will fit in a rocktour, but is not recommended. For the C&O and Allegheny trail, some slick, or less than full knobbie would work for me, but it's entirely rider dependent. I think, with the disc brakes, you could run 700c with some cross tires. My 26" rockhound can fit 42c cross tires, barely! This does raise the bb a bit, and probably voids any warranty with fox. The advantage to 700c is availability of tires compared to 650b; hopefully there will be more 650b tires in the near future.
I'm planning a C&O trip this summer; I'll look out for that rocktour