IMDB has the part of Avatar Roku cast. It's listing actor/stunt man Ben Cooke, who's most recent work as been that of a stunt man. He's also listed as being the fight coordinator for the film as well, so I don't know if this is a misprint or if he actually is playing the part. He's listed as both a stunt man and actor so it's possible.
This is the best pic I can find of him. Looks like he was also the stunt double for Anikan Skywalker. If this is true, we now not only have a major Fire Nation character cast as a white person, but we also have two white avatars.
It seems that there is also speculation that Randall Duk Kim has been cast as "Old man in temple" which in my head reads "Gyatso." They really don't want to give people names do they?