Just go ask for the owner and talk to them. You're going to get turned down 20-40 times before someone says yes. That is the only way. Give them a free night. And be prepared to give up a Weekend night if you really want to. My first restaurant ever, I worked for free for 3 months to prove to them the model. It was very successful so another restaurant stole me and offered me $30/hour. So I took it. Since then, that has happened to me several more times and I make a decent living doing restaurants and private gigs. I would never do magic at a restaurant that has less than 3.5 stars out of 5 and isn't already successful on their own merit. You are not going to make them more successful, get that idea out of your head. If you're really good, yes, you'll get a few regulars. You're there mainly as something extra for the guests, don't try to sell them on "upping their star ratings" on line or anything you can't actually prove. They'll know you're lying. So be brave, go door knocking asking for owners and managers, well groomed and in a suit with quality business cards and 3 of your best tricks in hand. DO NOT approach the restaurants you really want to do first. It's better if you can have a bargaining chip and that you already work some where. They are more likely to bite.Good Luck!

Just my opinion, but it's never a good idea. You come across as a jerk, and if you don't realize that you look like a jerk, then that's another problem altogether.

No, it isn’t, because your title didn’t mention performing for staff, it specifically said “Performing for PATRONS without management knowledge or permission.” Forgive me if I misquoted that.

Paul Diamond and Max Merlani could get away with it. Your mileage may differer

When I'm at a restaurant, the LAST thing I want is to be disturbed by anyone. Honestly, that includes the servers, but that's the nature of the beast.