How can I sync the documents of shareJS with a fileSystem
This is a bit late thought but you can still call getSnapshop method on the server side and dump that into a file on your file system. If it is not run locally you can create a tiny router with express on your local machine that listen for post request and you post the dumped file into the post request body on your sharejs server and then on your machine dump the post request body to a file, that should work.
Beware of security considerations if you use a auth system on your server.