Yes, it can be done, but just having it in the scene will increase render times, even if you don't see it. It may not be by very much, but it WILL increase render times.
I still say the 2-parts method would be better. Another advantage: You won't get confused as easily when trying to setup a shot. Yet another advantage: The Inside doesn't have to conform to the outside.
And I just like to point out that they DID what I suggested in FF7! Do you think that window to Tifa's room in the Lifestream/Flashback part was all one set/model? I seriously doubt it...
Sephiroth 3D @ School