When asked at the convention about a role in Episode 7, Ian said that it is unlikely, however he agreed to say that a spin off about the Emperor would be interesting.
So that casting rumor could be about one of the spin off movies that Kasdan and co. are working on. (by the Kasdan is not working on Episode 7, right? Is he?) (EDIT: IMDB: Kasdan: creative consultant. nevermind)