Do not forget the fine prints:
Unfortunately I don’t have much time left to spend on this project so I would have to leave this for a follow-on project. I think that it shouldn’t be much more work to do this as an extension, where we have RAIDZ Expansion for a while and then we add “RAIDZ Expansion improved space accounting”, and blocks written after that point would have the improved accounting (that is, assuming it’s possible at all).
Assuming that it is possible at all…