Of course, it's possible he could be playing a maester with a different name: Game of Thrones writers David Benioff and D.B. Weiss have been known to deviate from Martin's writings on occasion, particularly in the most recent seasons. However, if they stay true to the path presented in A Feast for Crows, Marwyn could end up being a pivotal character in the show's endgame.
If Broadbent is Marwyn, he could be around for a while. As Whitney points out in the article, Martin's book sees Marwyn depart the Citadel to find Daneys after he hears about her impressive fire-breathing army. In the interview, Broadbent reveals that he didn't get to do any CGI work this year, so it's unlikely he'll be hanging with any winged creatures by the end of Season 7. Instead, perhaps Season 8, likely to be the final Game of Thrones season, will find him mixing it up with the Queen of Dragons.