I think it can totally work. This is how I wound up with my first real girlfriend. She did the same thing (although over Facebook). And personally, I think girls being straightforward is not only hot but awesome on principle.
That being said; look as hot as you possibly can when you do it, since so limited an amount of contact will mean it will come down to looks. And have something ready to say after that for both an accepting and a rejecting reaction. There is a lot of potential for awkward. But you should totally do it.
Yes, that would be awkward. You need to "meet" him as he would any other girl. Just walk up and introduce yourself. Make sure somebody else you know is there so that you aren't going into a complete alien situation. After that, flirt like hell.
I will randomly approach guys and tell them they're hot, as long as they look nice and approachable. I've never had a guy that didn't at least smile and say 'thanks'. I've got guys' friends offer numbers (but I haven't taken them since they were not directly from the guy) and one time when I saw a REALLY hot guy, I couldn't let him go and handed him my number. We've been in good contact ever since. Go for it as long as they have 'open faces' and look approachable.