I think that my general approach would be to plan on cranking up the ISO setting, but to also perhaps see if the venue operator would be agreeable to letting me bring in a monopod with a quick-disconnect.
I'd also throw a sandbag in my trunk and see about using that if the facility's spaces has any nice horizontal surfaces...tables or railings...where you could use the sandbag for a stabilized shot. If no monopod or sandbag allowed, then don't forget that you can use a nice vertical surface to brace against, such as a doorway or column.
Lastly, don't forget that darkness isn't necessarily always a problem: it can also be an opportunity. As such, make sure to get some shots where there's some motion blur of the performers and/or audience members. Afterall, its a band gig, not a funeral parlor 😀
-hh