I explicitly mentioned Plan 9. Plan 9 doesn't do anything to make it fundamentally easier to write a filesystem, it just takes the (wise) approach of standardizing a simple filesystem protocol. (Which is a big help practically of course.) Nevertheless, actually implementing that protocol is still difficult. While it would be nice to represent a filesystem as a nested hashtable, 9P doesn't give you that for free: you need to keep track of fids you hand out.

Plash uses capabilities aggressively and only gives subprocesses access to what they need to access. But it doesn't make it any easier to write new applications which construct namespaces and proxy access.