O que você quer é chunkfs
:
ChunkFS is a FUSE based filesystem that allows you to mount an arbitrary file or block device as a directory tree of files that each represent a chunk of user-specified size of the mounted file.
Foi escrito para o mesmo propósito que o seu:
ChunkFS was originally written for making space-efficient incremental backups of encrypted filesystem images using rsync. Using the --link-dest option of rsync, you can create incremental backups from the ChunkFS-mounted image where any chunk that hasn't changed since the last backup will be a hard link to the corresponding chunk from the previous backup.