I thought the 50 move rule has been lifted for a few cases where a position is known to require more moves to win.

However, http://www.fide.com/component/content/article/1-fide-news/10... doesn't mention that, and even has the in some sense stronger "a game is drawn if any series of at least 75 moves have been made by each player without the movement of any pawn and without any capture.", which doesn't require either player to claim the draw, and, thus, cannot be avoided.

So, even if the players wanted it, a game can't be longer than somewhere around 96*75 or 7200 moves.