I would like to get a lot more interaction going on in Sarna.net as well! I like your idea of an online game.
What I was thinking of for the C-Bills, you would get paid each day for your current "rank" in the forums. You could send and recieve money to/from others. I will link this to the submissions where people can vote on other's mechs/units/etc, and they can get credits based on their rankings.
As for "using" C-Bills, I am trying to figure out how you could spend them. I will be opening an online store soon, maybe credits and/or coupons to online stores?