My suggestion is creating a google mashups site of where people can list where pinballs are located.
I've seen a number of user generated google maps mashups, unfortunately my skills fall just short of creating it myself...
I did a bit of searching... This site shows how to do this pretty much simply by entering the details onto a google spreadsheet...
http://mapalist.com/However, i'd assume there would be ways to make this more user friendly and useful...
Features could include:
Comments about the site
Comments about the pinballs
A time stamp or a time limit on additions to ensure information remains relevant
A rating system on the quality of pinballs.
Done well, I would use it often and would contribute. I'd expect operators would also contribute if it meant driving people to the pinnies...