We started a Beatles Night here in a pretty large bar about 5 years ago. We put in on facebook and asked for people to contact us if they would be willing to play several Beatle songs for a charity night. We have had such a response that we have to turn down people every year.
Some people come up for just two songs. Some are bands and some solo. I play in the band because I helped found the event.
Anyway. You guys should think about trying this in your town/city. I think it would work. You just go to a bar with a date and contact the charity that you want.
You'd be surprised how many people will want to be a part of it. Our town only has 20K people and it's just been a madhouse on Beatle night.
(If you want to know exactly how to go about it contact me. The guy who proposed it to me had nothin' when he started...just the idea, and we made a plan that worked great.)