On the surface this seems like a very good idea, with two (at least) fundamental problems: squatters and not being able to spontaneously decide to stay an additional night on your mooring. As for the latter issue, I don't know how to solve that one, other than, if you think you might want to stay another night, don't use a BoatyBall mooring.

As for squatters, I don't think it's realistic to expect harbour managers to be able to prevent bad behavior. So, I suggest implementing a fine system. The harbour manager can report the squatter boat to BoatyBall, who then fines the charter company $100, which they charge against the charterer's deposit. Rental car companies essentially do the same thing if you get a parking ticket in their car. Of course, the charter companies would have to be on board [no pun intended :-)] with this and warn charterers in their briefing. This wouldn't solve the problem that evening if somebody squats, but I imagine it would do a lot to prevent it.