Well. I'm not a professional or semi-professional, but if I was I'd never try to just turn up and pull that one. This is about knowing what outcome you want and trying your best to achieve it. I can honestly say that I've never turned up anywhere in the hope of having a go and not got to have a go. The thing with a harmonica player is that you can be discreet. You've got a couple of the things in your pocket and who would know? Just let it drop quietly to someone who looks like they might have the ear of the organiser that you can play/sing and wouldn't mind a chance to have a LITTLE go if they can fit you in. Mrs Steve has been known to act out that role for me, which makes me look even more 'umble. I suppose it's tougher if you're toting a guitar, which makes it looks like you've "turned up with intent." A bit of diffidence goes a long way, I've found. Put yourself in the position of a club organiser who quite likely already has a list of enthusiastic regulars who's then confronted by a drop-in with guns a-blazing and suffering from the "you do know who I am, don't you?" syndrome. Could be hard to be diplomatic...