> I couldn't find any information about the FS interface used in 
> NetBSD, and neither understand well enough how the "standard" FSs work.

Get -current and then start reading 

 man 9 vfsops

It references a lot other man pages for further reading. 

You also might want to look at sys/miscfs, you will find lots of tiny file
system implementations for various purposes. When developing a FS you probably
want to do it as a loadable kernel module, so have a look at sys/lkm/vfs too.

Martin