zpool user property is supported now. We can use this feature for the
cron scripts instead of abusing the zfs user property at root dataset.
https://github.com/openzfs/zfs/pull/11680

What semantics are you thinking for handling upgrades? This does not appear
to be a new zpool "feature", so we may want to support loading such a pool
on an earlier version of our zfs packaging.  How does this sound?

 - migrate the property if it exists (but do not remove the old, root
   filesystem one)
 - use the pool-level property if there's a conflict, but throw a warning
   if there's a discrepancy between the two
 - suggest people remove the old property if they don't need backwards
   compatibility in NEWS
 - in several version, start warning if the filesystem one is still around
 - several versions after that, stop even checking for the old property

Antonio