You can already do offline/lazy dedupe.

  zfs set mountpoint=foopy/foo /mnt/foo
  zfs set dedup=off  foopy/foo

  zfs set mountpoint=foopy/baz /mnt/baz
  zfs set dedup=on   foopy/baz

Save all your stuff in /mnt/foo, then when you want to dedup do

  mv /mnt/foo/bar /mnt/baz/

Yeah... this feels like picrel, and it is

  https://i.pinimg.com/originals/cb/09/16/cb091697350736aae53afe4b548b9d43.jpg

but it's here and now and you can do it now.