Cross bikes don't work all that great when the terrain gets really really rocky. Rocky meaning, you avoid a large rock only to hit another one 5 feet later. If you plan on riding this type of terrain with your cross bike it's going to be a constant battle against pinch flats or something worse like dented rims. I gave my local MTB series a shot last year on the cross bike, and gave up due to the afore mentioned problems.
There is something about mavic rims that all the other comapnaies are trying to copy. Open pros build really well and last a long time. I'm sure there are some folks out there that run open pros on their 29" MTB. I'd poke around on that subject as see what you come up with.