I may be off the wall with this, but would you be able to change their buy-in after the fact? My line of thinking is similar to how you put it - a raked bounty. You could (depending on version of TTD you're running) create a profile for it, name it raked bounties, and if able to, change their outright bounty purchase to a raked bounty purchase.
Again, I may be off the wall with this, but it is an idea.