Sure, the trails are open to all traffic. Just be damn careful out there. It goes without saying that everyone should always be careful on the trails, but mountain bikes can move pretty fast, and most of the trail traffic is slower than that. People aren't necessarily looking out for mountain bikes.
I've MB'd the trails at URE, years ago. Did it once and that was enough. Would be cool on a day when there was no traffic but like Roughshod said, be careful. Everytime I'd get going good something would happen to slow/stop me. Mid week mornings, maybe; Saturday afternoon, forget it! I can't recommend it as being a good place to MB.