I would definitely talk to them about installing some proper lights. And by proper, I mean anything to give you some ambience. Installing some coloured lights in the corners of the club, will great some awesome ambient light at a low shutter and costs next to nothing.
It's a win / win situation for both of you, as the photos will look better, and most likely attract more people to the nightclub.
Arguably this could be good & bad. If there are a few lights in there and it's relatively dark it means you have complete control over the foreground, and can go mad with shutter trails. Guess it's down to your style.
Though personally I agree with you, get some ambients in there!
I've just upgraded to a 5D MK III, and having had it's first test last night I can say I am blown away! The way it handles the highlights... just wow.
Tonight I'm going to give on-camera flash a go again (last year I've been TTL cabled with off camera) as Srika seems to have some sort of golden technique that I desperately want.