There is this idea.
Not mine, I just saw it somewhere else.
But great.

Every user of the forum has a BTC balance, which is like an online ewallet where he can store his bitcoins (by sending them to the forum's wallet up front, or to some payment processor, whatever)
So you can deposit bitcoins from your account or withdraw them - might be automatic, depends on the scale.

And additionally, next to each post, there is a button - when you click it you send 0.01BTC from your wallet to the author of this post.
This is great idea - yous should make it. I can help you with coding if you want