Not the entire boot volume, but close. People have put user directories, etc. on ZFS drives. Google "MacZFS" for an older version (ZFS v8) or "Zevo Community Edition" for a more modern version (ZFS v28). There're active discussion boards for both.
Ah, my mistake. Rather a pity too... they actually had a product that was usable. I suppose the biggest challenge for them was "who will buy this" given that it was targeted for people with large storage arrays attached to Macs. It's a bit of a niche; perhaps people in the media world would bite?
Not boot volume though.