If you have steam, you could use a dummy key. The game recognizes it as valid, but it acts like a banned key and won't let you online. So you can use add-ons but- only in singleplayer/lan mode.
Then I guess you could use tunneling software such as hamachi or evolve to connect with your friends over virtual lan. So you have some sort of online functionality. But that's alot of work.
I don't know where you could get a key like that, but it certainly wouldn't be publicly available on the forums.
I should test that to see if it works. It would be comforting to know I can use BL even if the authentication server goes down.