I feel sure HBO brought this up in their negotiations. You think he provided them with a bare bones outline of how its supposed to end?
No. If I was in his position, I would entrust an author or group of authors with finishing the story. I'm sure he has his ending written down in a very broad sense so I would make that available to them only at my death. From there they take over the story and finish it with his take on it in mind. HBO would not be making the decisions and I would not trust them to not leak the ending.