Is there a minimum weight limit for mountain bikes for USAC riders who are not UCI? Basically the title is the questions, but to repeat is there a bike weight limit for riders who belong to a national association such as USAC, but are not riding for UCI points or registered with UCI? Or is it the same as the UCI limit?
My understanding is that UCI and USAC use the same minimum weight limit for bikes, but for USAC this only applies in certain circumstances. Taken from this document from USAC:

Bicycles must meet current UCI technical regulations at
events that select 17-18, U23 an
d elite riders for international
competition or national teams. A
ll bicycles used in Federation
National Championship (for age
17 and older riders) and NRC
races must comply with
the current UCI regulations
.

In other words, if the event you're racing in is not a qualifier for an international event or for a spot on the national team you can disregard the weight restrictions.
For completeness, should one of the above situations be applicable to you, my understanding is that the minimum weight for your mountain bike is the same as it is for your road bike, which is 6.8kg/14.99lbs. However, I must admit that I could not find a black and white answer to confirm this assertion. I think this is largely due to the fact that the nature of mountain bikes is self limiting in terms of this restriction. Unless you have something especially exotic in your stable you're not going to even approach the minimum weight that is explicitly defined for road, track, timetrial, and cyclocross bikes.