Well, before start the project, I'd like to make same tests to see it this solution is feasible; then, you could watch both captured versions (1.95:1 and 2.35:1) to set the margins - they should be made pixel perfect, so it's a HARD and BORING work... (^^,)

Here you are an example; green is the theatrical 2.2:1 (70mm), red is 2.35:1, blue is 1.95:1:

It is still possible to do two versions, one theatrical 1.95:1, the other extended, 2.35:1 (with or without the help of the 1.95:1 version to improve final resolution). Plus, there is a third possibility, "a la Batman"... use the 1.95:1 version "as is" for the theatrical scenes, and 2.35:1 for the extended scenes.

If you feel corageous enough to try to accomplish this task, you are welcome!