If you absolutely have to have him autograph you (as opposed to using a picture of his autograph for the tat guy to work off of) you're probably just going to have to get really lucky and bump into him outside a gig (which will probably not happen) or when he's out and about in a certain city.

You could try and get some details for his management and e-mail them and see if they have any ideas for meeting him quickly at some point but I doubt they'd go with it.