I wouldn't even worry about it. I leave after openers all the time.
Saturday I was hired to tape the opening band at a club here in town. The drummer for the headlining band came up and checked out my gear during the tail end of the set and asked what was up. I told him I was hired to tape the opening band. He said, "oh OK. Are you sticking around for us then?" I said "I hadn't planned on it." and that was that. No grief, no guilt trip. It's a club, after all.
Had he offered to pay me for my time like the opening band, I probably would have stayed. It wasn't in my plan though, and it was going to take some $$$ to change that plan.
I agree with previous posters - most people won't care if you leave between bands and you're fully within your rights to do it and no-one there should be able to say boo to that. At club shows that happens all the time anyway! Tearing down in the middle of a room in the middle of a set though, that is bad form (although I have done it before and would certainly do it again if I had to). Try to avoid it if you can.
Dirk