The octaves might be tricky. Since I can't do what I would normally do to get a choir to work better (dual-layer the notes, i.e. having one at a high octave and one at a lower one of the same key), because then it would sound like a choir of singers and not a single one (and I really don't want to do that), I might have to resort to using strictly the vocal samples from the respective performances. I don't personally have a problem with doing that, but I don't know how well it would go over with people if I used the game's original choir along with high-quality orchestral banks. Of course, I've done it with a few other tracks, but those had very specific sounds that couldn't quite be replicated as easily (i.e. the Devil's Lab noises, Chocobo synth noises, the game's amazing rock organ (no other organ sample/soundfont I have captures that "feel" as well as the original), etc.)...maybe I can get away with it.
And I'm not sure. I'd like to do the unreleased songs, but I might not be able to. I highly doubt anyone's sequenced a MIDI of them (that's entirely accurate), and I'm still looking for SPCs. We'll see, but I am trying.
EDIT: Ha, I see I'm not the only person you've asked that question of.