The amount of new material in Coda is negligible. It is a handfull of shots, less than 30 sec,s in total...pitiful money grabbing exercise IMHO.
I did full frame by frame, scene for scene comparison of Part 3 and Coda. My La Fine edit is as close as you can get to an extended part 3, and I added a new scene extension using dialogue from deleted material on the DVD and repurposed HD images to incorporate it. I suspect that there is not much extra material to use unless its more close ups of Mary making strange twisted smirks to camera in lieu of acting....
As far as extending the other parts any further, unless a copy of A and B rolls in good condition is located, given how damaged the original negatives were when the Copolla restoration was made, all that is left is a VHS of the assembly cut with a HUGE timestamp in the center of the screen, and that is unlikely to see the light of day unless it is incorporated into a documentary. The issue with that I think is that admitting that those gems of Vito growing his Empire (De Niro) and Michael finding his feet, could diminish peoples perception of these movies. There will not be a "Donner Cut" so to speak of The Godfather. Even the assembled negative dupe that was made to generate the chorological versions for TV and VHS release is in such poor condition, that it may now all be lost to vinegar syndrome.
Rejoice that we have what we have.
(now, where is that original cut of The Magnificent Ambersons)
ps I'm convinced this will turn up one day in a mislabeled can in a French or South American archive (before I die with any chance).