Yes, that is a brilliant idea. Minimum investing or a payoff from the bounty project seems really good, but the thing is, it hardly seems to be practical. Even if a few bounty hunters agree, there will always be those who will be antagonising your idea, and those who will be totally frustrated and you cant do anything about it because they have freedom of speech and freedom of making their own decisions.
Forum members can come up with rules for bounty campaigns, discuss them and if necessary vote. Ready coherent option, you can grant administrators of the forum. And if they approve them and publish them as mandatory, then new projects will somehow be obliged to adhere to them.