As noted in a recent commit, it’s possible to set up a HAMMER2 /usr/obj and survive a buildworld. That’s good progress.
Note that this is basic work, so features like multi-master and deduplication are not present yet, and it’s still work in progress, so don’t try HAMMER2 unless you like losing data. Watch the branch for changes, though.
(I’m going with “HAMMER2” for the name.)