Martin has already told the two writers who adapt the books to television who ends up on the Iron Throne at the end of it all. It is in this
interview with the three of them. Now, whether he told them everything - how it gets from where it's at now at the end of Dance with Dragons to the end of A Dream of Spring - is anyone's guess.
I would suspect he hasn't shown all his cards simply because of how he writes. He does not do linear writing - meaning he doesn't do an outline and follow that. He turns his muse loose and writes. If it feels right and fits, he keeps it. If it doesn't then he has to go back and rewrite. Which is why he takes so bloody long to write a book. Well, that and the fact he is working on other projects at the same time as writing Winds of Winter.
Last interview I read said Winds of Winter will most certainly not be out before 2015. One more year more or less :(
Addendum: You can read excerpts from Winds of Winter HERE
!! Nothing like a teaser to wet the appetite and make you whine for more! LOL