File overlays? Some old OSes had them. They could create a different "view" of files. I bumped into finnix a while back, which will create a filesystem with overlays for a mounted cd
See
http://www.finnix.org/Overlays
However I don't think it is precisely what you want. What you want sounds like at least a specialty filesystem, if not kernel changes... can you tinker with mmap, you can mmap individual pages of a file?
Try a google on unix filesystem overlays. or mmap file overlay