I agree that there was an outline. Perhaps it was written. Perhaps it was offered verbally. Certainly Moffat told Eccleston, "Here's what I see your character doing. Here's how I see your character fitting into the story." Certainly Moffat tried to sell Eccleston on the reasons why he, as an actor, would want to return to this role. If Moffat tried to sell Eccleston on the nostalgia factor, it was a lost cause right from the start.
But I doubt there was a script. Moffat isn't a fast writer. I can't really see him spending time on a useless script.