The position of the FAA regarding where the overtime money goes (ie, that it's used to pay controllers at the home facilities who cover the absence of the special event controllers) comes straight from the mouth of an FAA "OSG Airshow Specialist". I've been in discussions with him for months, regarding a reimbursable agreement for the temporary tower at the Copperstate Fly-In. The way he explained it to me, doing it that way is for the show organizers benefit-- if, after the show, it turns out that the absence of the controller from his home facility didn't result in the need to call in OT, the show organizer will get a refund.