I was talking to Marc Gravell on chat and he thought this might be worth mentioning here to see if others would be interested in it. It could be implemented as a simple grey star in the Gravatar popup box, when clicked it goes yellow and that user is added to your "Friends list" (sounds a bit care bears and unicorns?). e.g.
Now, in the lobby Marc would appear in my Friends list and I could access this Gravatar box from there without having to join a room he's in or search for him using the search box.
Another possible implementation might be to keep track of who the user has been @-mentioning and populate the list as a "recently chatted with", as Jon B mentioned in the comments.
Related (just in case you wanted to connect the two somehow):
A "friends list" on Stack Overflow would be nice