I ended up just adding the PGmodule to the app instead.

You used libpq to write such a PGmodule?
I worked with PostgreSQL for 6 year... I find it wonderfull, and I would use it as the db backend for any opensource application I would write.

I realise that exporting and mounting would be facilitated by the FS layer but I didn't have any reason to exploit such availablility.

Basically I got as far as saying to myself - this will buy me nothing more than I have already so I'll just stop, one fo these days I might do the FS side of it, maybe.

I'd like to know how do you would map the operations to the filesystem.

Query result in a filesystem hierachy would be a pain... xml would be better, since you could transform it quickly.
BTW, there should be an xmlfs project waiting somewhere...

Giacomo