Even if the CGI was good enough (which, judging by Tron Legacy, it isn't yet), I think it would be kind of missing the point.
The fun of the idea is getting to see the actual, original actors together in the same room.
It's science fiction... science fantasy... whatever label you want, there is a creative solution as to why the "Doctor" has aged even though he "regenerated." I thought it was brilliant hand waving in Time Crash.
It doesn't matter that they've aged--of COURSE they've aged, they are REAL people. I want to see them performing together.
I just don't get the overwhelming need for veracity in a show like this. It's about a guy who travels in a box through space and time. Some things you just accept.
And why not accept a slightly older 6th Doctor?
BUT--I do wonder if that's the direction they are going to go. Do you make a love letter to the fans of the series as a WHOLE, or make it more about the show as it is now?