I built https://github.com/zimbatm/h to satisfy the same need but it's a command-line tool, not a FUSE filesystem. In some way I prefer it because transparent network access doesn't work really well with failure modes.

Usage: `h <owner>/<repo>` clones the repo, if it doesn't exist, and takes you there.

This and hub are integral to my developer workflow:

    $ h some/repo
    $ git checkout -b some-branch
    [hack and commit]
    $ hub fork
    $ git push -u zimbatm
    $ hub pull-request
    Done