I've read the Potter agreement. There's absolutely nothing in it that says Potter cannot be used for HHN. It's a fan rumor/myth.
The reason we haven't seen HHN return to the Islands since Potter, if I had to make a hypothesis, is because JK gets a take of every person that walks through IoA's gates. With HHN in USF, she doesn't get a take of the profits.
So when 2.0 open Universal really wont have much of a choice but to pay her then right?