After 3 afternoons of work during my recovery period and a couple of hours today, I got the first VFS operation to work! :)
So.. if you compile the latest development code from the Mercurial repository, you can now use 'df' on ZFS filesystems in Linux (but I don't recommend you try it on filesystems you don't want to corrupt) :p
It was a lot of work since I had to make zfs_mount() work, which depends on a lot of other code (I even had to import the range locks code).
The VFS is quite complex -- I still don't have a firm grasp of the whole VFS and vnode allocation/deallocation/reference counting, so my code is very messy, and there are still a lot of bugs :p
I also got a little disappointed because FUSE doesn't support remounting, so you won't be able to change any mount property on mounted filesystems (like for example, 'zfs set readonly=on pool') - you'll have to unmount and then mount it again..
Next step is to fix bugs/clean-up code and then implement readdir and stat (in order to use 'ls').