Can I run a wheel with a 130mm hub on a mountain bike with 135mm spacing? I'd like to build a wheelset that would work on my cross bike or my 29er, assuming I had disc brakes on the 29er and rim brakes on the cross bike, is there any way I could use a 130mm hub (that fits the dropouts on my cross bike) and make it work on the 29er with 135mm spaced dropouts?  

The problems I would see would be that the disc rotors wouldn't line up on the 29er.  Could I use spacers to make the hub fit on the 29er or would I need to use a 135mm hub and spread the chainstays on the cross bike?  Would either of those options work?
I've respaced a steel road frame from 130mm to 120mm for a track hub. It's super easy, although Mr. Brown recommends not doing this for a change as small as 5mm.

I'd try a 135mm mountain disk hub with a 700c cross rim, this gives you the most correct breaking surface on both setups.

The advantage here is that when you come to put the 130mm road wheel in your cross bike, it still fits. Although your cross bike MUST be steel and not aluminium of carbon.

One last thought, this whole scheme is going to be a bit of a bodge however it's done.

I'd be more tempted to have a pair of shared road wheels, where the stresses are much lower and a dedicated pair of off road for each bike. A 120mm/700c cross set, a 135mm/29" MTB set and a 135mm/700c shared set with slicks.