I’ve actually seen Interstellar on blu-ray recently. (wish I’ve seen it in imax, I wouldn’t care if it was digital or not.) I am sort of mixed on the shifting aspect ratios with that film, however I support it in the end.
When it changes during something simple, like characters talking during a car ride it is somewhat distracting, but I can deal with it.
It worked perfectly with the scenes in space, as the shots inside the ship were cinemascope, while outside they were imax. It created a definite difference between cuts, and just looked good. It even worked during the scene near the end, but I won’t spoil it for the few of you who haven’t seen it yet.

OT: Taking the imax screens from the 3D blu-ray would be easy. All you would need to do is:

Split the 3D blu-ray into two files, one for the left eye and one for the right.
See which one matches with the 2D blu-ray.
Take the imax scenes from the side that matches and splice them with the 2D blu-ray footage.

It would be even easier if the full imax shots are special features.