I did a very small prototype of this a year or so ago. My interest was in providing a simple get/put interface to the JCR repository - basically making it look like a filesystem with xml documents instead of an amorphous collection of nodes. It basically worked and wasn't too difficult, but I had to move on to another project before I got it to a reasonably polished state.
Conceptually the idea is sound and the mapping isn't that difficult, but there are some things that make it tedious (such as the tendency of jcr to use a billion different namespaces).
