Johnny Ringo said:

Getting back to the Imax stuff, I wonder if it would be much hassle to cut out those sequences / re-sync with the proper surround audio and just present them as a standalone feature.

It's a tremendous hassle. There are a lot of missing frames in the commentary. Additionally, some of the BTS windows obscure footage that can't be comp'ed out with the 1080p version of the film.

I've got a rough version done (I don't think the surround sound is in there), but it's cut together from front to back. It's just impossible to be incomplete unless there were some serious effects genius around these parts.