Would it be possible to get some kind of bronze badge or bit of flair to show if someone has attended one of the Stack Overflow meetups?
While I think it's good to encourage the meetups, I don't think it's appropriate to provide a badge to reward activity that doesn't actively contribute to content creation/moderation/improvement on this site.
From the FAQ:
There are some current exceptions, like Quorum (One post with score of 2 on meta) and others like Announcer and Booster (Shared a link to a question that was visited by M unique IP addresses in N days), but those are also about contributing back to the community as well.
Maybe they could goad some of the location-based checkin providers (foursquare/gowalla/et. al.) to provide a badge though, since those are ones that actually track location checkins. But given that it's lots of various locations across the globe (some of which haven't even been decided yet), I think that'd be tough to do as well.