Not really he possessed someone's body in 'The Keeper of Traken' one of the final 4th Doctor stories. As for Timelords/ladies choosing their appearance, the 2nd Romana played by Lala Ward did just that. She'd been cast in the final story of the previous season 'The Armageddon Factor' as Princess Astra and in the opening episode of the following season 'Destiny of the Daleks' (briefly revisited in last week's Dr. Who season opener) the Doctor thinks she's that character before she explains that she's Romana and she's regenerated, choosing that face. Also much earlier than that in the final 2nd Doctor story 'The War Games' the Timelords present the Doctor with a choice of faces for his next incarnation.
I'm going to go with the Romana example being the thing I was referring to. Sounds like it. I remember a Who fan saying something that they could choose their bodies.
I also consider the McGann idea very unlikely as well. But theoretically, if there is precedent for a time lord to choose his appearance, and Tom Baker certainly implied that the Doctor is going to do just that in the future, wouldn't a former Doctor returning to the role actually be possible?
It doesn't sound like anything BANS the idea, not that they will actually do it.
Getting back to McGann--even if he just took a role in another show, is it a British show or a US show? If it's a US network show, then I can see it being very difficult to return to Who, but if it's a British show, given that they have such short seasons, why COULDN'T he if he wanted. As recently as last June, he stated he would return if asked.
This might be moving off topic, but there really is no reason he can't return for a true multi-doctor story. OR, he could even have his own special, or a spinoff as the 8th Doctor.
If they can do Class, why couldn't they run off 10 episodes of an 8th Doctor story? Even better, make an arc, and tie it into something the current Doctor is doing, where they finish both stories with a crossover.
One thing about these writers, I think they are good enough to pull it off.