I support the idea of a separate subreddit for posting meetup posts, with its own segment of the sidebar so it can be monitored during browsing of the rest of the site.
I think this would be the best solution. To compress the information further, it would also be good if the sidebar block only showed meetups yet to happen, and all others relegated to a "past meetups" link. As of today, April 11, it might look like (but with nice fonts and everything):
Meetups
Edinburgh LW Meetup Saturday April 16th
Ottawa LW Meetup Saturday April 16th
past meetups
That would need some automatic way of discerning what the date of a meetup is.
I've been browsing LessWrong less often as of late, and I believe that one of the reasons is because I don't want to get all excited all over a new insightful LW post, only to discover that it's a meetup in a city that I'm not even in.
And for newbies to the site in particular, they're not going to see all the awesomeness of this site if they simply look at the frontpage and think it's a community just for meetups.
I'm sure meetup discussions can go somewhere prominent. Maybe a separate section can be devoted to them.