Are velocity A23 rims suitable for building a wheelset for a 29er Mountain bike?
I've briefly ridden A23's on a road bike, and while I think they'd be OK for cyclocross I did find them a bit flexy (I am ~220lbs).  I've spent significant time on the delgado cross rims, and I found the A23's noticeably less stiff.  I think you'd want something a bit beefier for singletrack riding, especially if you are hard on wheels.  

In your situation I'd probably want more of a mountain bike rim that's skinny enough for the cross tires  than a road rim that might be strong enough to stand up to mountain biking, but imagine the lack of good mountain bike options with v-brake compatibility does make the road rims more attractive (along with their light weight).  Guess this all depends on your weight and trails you ride though - I'm a pretty heavy guy and ride in eastern PA where we have a lot of rocks/roots.  A lot of people seem to like the mavic open pros as a 29er rim and they don't look much beefier than the A23's - I always assumed these folks weighed 120 lbs and were riding really smooth singletrack but I guess I could be wrong.

If sticking with Velocity, maybe the Dyad would be more up your alley?  They look a bit like A23's on steroids.