Might be a stupid question, but is it possible to play CM 99/00 online against others and keep an on going league?

I don't think so...if you want people who know these answers for sure, go and post it in the CM 3/99-00 forum...

Hey, yes it is possible to play it online. Just do network game and check your IP number to play against friends.

But it is not possible to have a league within the game I think, you would have to keep the stats somewhere yourself...