Originally posted by seven:

The problem with the beta opt-in solution is that default_old also gets updated. What is working for me is to save a copy of the game to a location outside of steams game folders, and then run the game from that location. I haven't noticed any problems doing this. I don't know if works with MP though. And you don't get the steam overlay (unless you add it as a non-steam game) but I don't really care about that.

Ahha! Nice. I've always wanted to check if Steam has anything to say about that. Good thing to know that actually works.