Yeah, always have a decent d.i. on hand for these occasions, I always have my SansAmp BDDI in my bag.
I've had to use backline rigs, most often lucky to use an SVt / 810, or a 410 or 610, but once I played at the Charles Playhouse in Boston in the downstairs club during Blue Man Group intermission, and had to use the house amp, a beaten within an inch of it's life Traynor. Thing barely functioned, if at all.
If you're a club demanding acts do not bring their gear (space constraints in this case) and use a supplied backline, at least make sure it's a decent piece of gear! I mean you're making money from the performance, reinvest already.