Just to summarize all the info so far:
1. Shoot at EI 3200, develop for EI 6400.
2. Xtol is recommended for less visible grain.
3. Watch the camera's meter when there's backlighting; switch to spot metering if necessary.
4. Under/overexposure is the enemy.
The event's on Friday night but I like to prepared so I'm arming myself with all the info I need. Thanks for all your help!