You could have an opt-in so people could participate in "neighbourhood celebrations" and all show a ribbon for a particular event, say, TimBL's birthday or something, like Google Doodles.
Either way, this is bloody awesome, good luck with it all.
And no advertising on sites, ever. This is our golden rule.
My goal is to never make any changes to other people's sites. The only exception I'm giving myself is for an extreme emergency, such as a major security problem. It's never come up, I doubt it ever will. During the SSL transition there were a few sites I helped convert to SSL, but that was with permission from the site owner.
I remember the watermarks. They sucked.