I am sure that even after you found out all her personal information that she couldn't possibly want you to know, I doubt that you'll learn anything new. If she didn't tell us in the book, she isn't going to tell us in the letters or whatever. Time will reveal all. And I hope IF you do contact her, don't put my name on anything because I really don't want to get arrested.
Right, how's this for a theory:
Maybe that veil leads to another dimension or something, and in that dimension, Harry is dead, and there is only one dimension where Harry is alive. So, Sirius gets out of the Dept. of Mysteries (disapparates) and then finds his mirror to try and communicate with Harry. Since all the other Harry Potters are dead (not survived the encounters with Voldemort), the only window the mirror can open to is the one in which Sirius was before he fell behind the veil, the dimension where Harry Potter is alive. So, Harry one day is sorting out his school trunk and remembers his broken mirror, and notices that Sirius is in there! They communicate, and then so on ... Now, criticise this all you want, it's a very bizarre theory, but remember: Nobody knows! 😎
The case AGAINST Sirius being dead:
First of all, Miss Rowling has a tendency to introduce something in one book, which seems quite minor, which then becomes a centrepiece of the storytelling in the next book (such as Azkaban, for example). That archway is bound to come up again.
Secondly: The Thestrals. The room in which Sirius was supposedly killed contained only people who had already seen death. That was the first thing that struck me, when Sirius was hit by Lestranges magic and fell through the arch. No one else was killed. Only Sirius. So had, say, Hermione been in the room, and Sirius had really died, it would be a matter of reading in the next book if she could see Thestrals or not. Rowling is careful to make sure only Aurors (who’ve seen death) and the Death Eaters (who’ve dealt it out) were present, along with Neville and Harry.
Thirdly: In the very end of “Order of Phoenix” we hear of Dumbledores plan to keep too much weight of Harrys shoulders. We learn, that Voldemort abused Harrys love of his godfather to lure him to the Department of Mysteries. Perhaps Sirius has been asked to pretend to be dead if he got the chance, so that Voldemort could no longer use that against him. Harry can’t do Occlumency yet, he’s vulnerable to attacks from the Dark Lord.
And finally: Where’s the body? Why take us to the room, make sure we learn that the Arch-way is strange (Death?), and then toss Sirius through that, if it was easier to hand over a body with no lifesigns. Sirius has spend more than a decade in Azkaban. He could probably endure quite an ordeal.
I'm 99 % sure he'll be back, when we don't expect it.