I doubt if Jack would have had a written will, at his age.
I think, him telling everyone he wanted his ashes scattered on Brokeback Mountain, was his way of making sure, that in the event of his death, Ennis would know, what the meaning of this request meant. How important HE was to him, and how the place that they met had so much meaning to him. He would have also known, that if his ashes were on Brokeback, then Ennis would have been free to go there without any questions asked.
I guess too, it was his way of saying that he would be there where he felt the greatest amount of peace, where he could be who he really was.
Maybe he also assumed that Brokeback, would be the place that Ennis would request his ashes be scattered as well, so that at least in death they would be together. Of course, Jack did not know he would die before Ennis, so, it is doubtful that Jack would have forseen Ennis meeting his parents or talking to Laureen. I just think that he maybe hoped that they both would eventually be scattered on Brokeback, regardless of who died first.
He also would have known, that if his request was granted, and Ennis was still alive, then Ennis would be involved in the scattering of his ashes on Brokeback.