I'm not a signature guy either. When Jim from The Sanctum was taking my Dirty Harry to Pintastic for the Silverball Tournament he asked me if I wanted Barry Oursler to sign it. I said "Why?".
As for signing the center of the playfield? How annoying. A mod that blocks artwork and you'd have to seal or you'd end up with a real spotty mess.
Have them sign a $1 bill, tape it in the coin box and say you made him pay to play their game. No one rides for free.