Oh, the mischief Froggy wreaks!
Turns out that that last post from GRRM is not a casting clue, as confirmed by Amy Joyce Hasting herself. According to her, that cover (created by Poppaea Bicknell) was for a prop for the film The Callback Queen. Morgane Deare played Horatio King, a bestselling fantasy author whose books were the basis of a big production…
And it turns out that he and his film were directly inspired by GRRM and HBO’s Game of Thrones, which is why George shared it. As to the mischeviousness? Froggy the Gremlin is a troublemaker.