A new casting clue
from GRRM seems to confirm that Gwendoline Christie, long a fan favorite for Brienne, has been cast in the role.
(Christie was in the music video for a song on the same album as one about Isabella of France, called the she-wolf, has been on stage at the Barbican theatre, and worked with an Australian photographer on a project called Bunny Nose. And the post with the clue is titled "I'm Blue.")
An alternate theory is that the post refers to Sophie Marceau, who played an unusually sympathetic Isabella in Aussie Mel Gibson's Braveheart
, and has had films at the Barbican. If so, she's presumably not Brienne.