It would probably be easy enough to write a scene which involves The doctor having to pass through a place such as the national gallery again, and bump into 'the curator' along the way, so I wouldn't say unfeasible.
My only issue would be that they asked him to appear in the 50th because it was supposed to be a special celebration of 50 years, so for him to have further appearances in normal episodes would kind of diminish the specialness of his appearance in the 50th. That being said, if it was revealed tomorrow that he was doing another scene, I wouldn't exactly be complaining.
I just think it's a shame he'll probably never pop up in another multi doc episode, and as such never meet 10. wish it had happened in the 50th, as despite individual opinions they are overall regarded as the best doctors of Classic who and the revived series respectively, so what a shame that the two greatest doctors will never meet.
