It’s been awhile, but George R.R. Martin has provided a casting clue on “Not a Blog”. Given the end, it’s clearly for the role of Mace Tyrell, Lord of Highgarden…
... and some quick work on the Internet already points to British character actor, writer, and director Roger Ashton-Griffiths, who fits all the particulars in the clue to a T. He’s appeared in a wide range of films and TV programs, including Brazil, Gangs of New York, The Tudors, and A Knight’s Tale, and will be appearing in Grace of Monaco in the role of Alfred Hitchcock.
Update: And Martin has since updated his post, confirming that Ashton-Griffiths is indeed playing Mace Tyrell.