and 12/13 are handles to other objects. So to write a host filesystem, you'd have to write an OSD responder that creates those "directory files" on the fly based on the filesystem. Then you get races if the guest and host both modify a directory at the same time, you'd have to cache what you last told the guest, and then see what modifications it made, to apply those modifications to the filesystem.
All in all, I think a different protocol would be much easier.