It would never work. People give far too much credit to computers when they propose things like this. They assume computers are magic and can do anything, but that's very wrong. No matter how photorealistic a CGI creation is, it's still reliant on a human actor (and human animators) to give it voice and movement and personality. The computer is just a tool operated by human artists, interpreting a human performance. It might look like Montalban, but it wouldn't sound like him -- the technology to computer-simulate a natural-sounding human voice is still years away at least -- and it wouldn't act like him. It would just be an impersonation, no matter how much CGI trickery you threw into it.
You see, Brando in Superman Returns was not a full CGI creation. They took footage of the real Marlon Brando that was cut out of Superman II and modified it with CG, but the voice, the performance, was the real Brando.
Besides, why would they bother? They've recast everyone else -- they even had two different actors as Spock in the same movie.
Not to mention that it's still just a rumor that Khan will be in the movie at all.
