Well, they could do what was done in the past with Seuss Landing: turn off most of the lights, populate it with hooded scareactors, and play the music backwards. I thought it was appropriately creepy, while still within the realm of the agreement regarding using the island for HHN.
In regards to using Potter, whether its mentioned in the contract or not, I would think that Rowling's approval will still be required. I wouldn't mind seeing the island open in its normal capacity during the event (I LOVE WWoHP at night), but I can't help but wonder how she would feel about it.