This happened a bit last year, when they inserted scenes such as Drogo killing Mago, into GRRM's episode. However, this year was a much more severe rewrite as the changing budgetary restrictions meant that had to keep rewriting the episode.

For example, originally they had no extra money so outlined the episode as a near-two-hander between Sansa and Cersei with people rushing in with battle reports. They realised this would be awful, so negotiated more money with HBO. GRRM then wrote a version of the episode with the effects toned right down, but I believe with several extra elements (the chain included, IIRC) retained.

The producers then rewrote THAT version with even the watered-down effects sequences reduced to not very much at all, a bit more than their original outline, but not much. Then they got the more money they had asked for and then (at the last minute) they got Neil Marshall to direct. Marshall, who is a master of taking tiny amounts of money and making it go a long, long way, was able to get more battle scenes out of the budget then they had planned for, so they then had to rewrite the episode again (on the eve of filming) to accomodate that.

So in short, whilst some of GRRM's script is in there, a lot of it is David Benioff and Dan Weiss. In particular, they have severely changed and weakened the Sandor/Sansa stuff throughout the whole series (something that I know GRRM was concerned about in the first season), so I'd be surprised if that scene wasn't written by them.