It already exists, it's called git annex. It uses git to keep track of where files are (but doesn't check them in), so you can just do:

  $ git annex add myfile
  $ git annex copy --to=NAS myfile
  $ git annex get myotherfile
  $ git annex drop --from=NAS myotherfile

If the NAS has rsync support, you can add it with a single command:

  $ git annex initremote NAS type=rsync \
    rsyncurl=rsync://example.com/myrsync encryption=<gpg-email>

http://git-annex.branchable.com/