Have a (few) friend(s) hold a flash with a tight grid on a boom. Sync with radio. That way, your flash will look like the spot light. You can crank up shutter speed and nobody will probably get pissed of, except the people on stage. You wont need to gel it, unless you want the flash to look like tungsten, or someway in between tungsten and flash (which usually is pretty nice). Loads of power needed as polyglot (excellent reply) said, specially with that slow film.