"I haven’t had any information [on the 50th] until up just about right now and all I can say to you is, we’re discussing things. That’s a total exclusive! Even yesterday I was saying ‘I have heard nothing.’"
He later tweeted that he "didn't say in what capacity or what medium so stay calm and carry on."
Freema Agyeman previously unofficially confirmed some sort of conference for the Anniversary, so this may be a false lead, but considering someone's probably coming back for the 50th, it seems likely one of the most popular NuWho companions will be returning.