If the Gate is not willing to turn the car around, then I would suggest a $100 fine on the car per stowaway. Paid by the stowaway, the driver, or the passengers, who cares; they're all guilty, spread the joy.
You're speaking of the '*art tax' which has been in place for several years now. I believe Mr. ^!%@$#&$* came up with that one.
* aka 'dumbass tax' which is a sliding scale 'suggested' contribution above the price of the regular ticket price. Usually falls just below the BLM fine of $250/person for violators who try coming into the event by way other than the established gate route AND it won't go on your record. NOTE: credit cards gladly accepted. Monies collected DO in fact go back into the event/project.