Not about unused material, per se, but in line with the subject of unofficial, private 'fan editions'...
I remember PJ saying that with the advancement in standard consumer-level tools and technology it won't be long before fans could make their own versions for home viewing, such as recutting the scenes so that they are grouped more in the order of the books, for instance.
That wouldn't even require any reordering from the original form on the media itself. Just need a playback overlay where tags (unique pointers to time offsets) could be set at specific points, and a master list indicating in which order the segments are to be played. On a whiteboard the design looks pretty simple (basically, it's just metadata indexing).
The high-capacity DVD formats both had varying support for this concept. But it won't be long (surely, before the Hobbit is in theatres) that we'll be storing everything on solid-state and static drives (both HDDs and discs will be dead). Then we'd only be limited by software (and licensing restrictions).