[~] uname -apKU
FreeBSD harbinger 13.0-CURRENT FreeBSD 13.0-CURRENT #5 r364030: Tue Aug 11 07:15:59 UTC 2020     root@harbinger:/usr/obj/usr/src/amd64.amd64/sys/GENERIC-NODEBUG  amd64 amd64 1300105 130
 [~]
 [~] pkg query '%o %v %R' openzfs openzfs-kmod
sysutils/openzfs 2020080800 FreeBSD
sysutils/openzfs-kmod 2020080800 FreeBSD

 [~] bectl list -aDs
BE/Dataset/Snapshot                          Active Mountpoint Space Created

r364030
  zroot/ROOT/r364030                         NR     /          15.1G 2020-08-11 08:25
  r364030@2020-08-15-20:40:36-0              -      -          688K  2020-08-15 20:40

r364030-OpenZFS
  zroot/ROOT/r364030-OpenZFS                 -      -          284K  2020-08-15 20:40
    zroot/ROOT/r364030@2020-08-15-20:40:36-0 -      -          688K  2020-08-15 20:40

After activating r364030-OpenZFS BE and mounting on /mnt, I edit:

- /mnt/boot/loader.conf to load openzfs_load="YES" and comment out zfs_load="YES"

A reboot fails during the boot process dropping the system at the mountroot> prompt

On screen I see:

Loader variables:
vfs.root.mountfrom=zfs:zroot/ROOT/r364030-OpenZFS

Some options are displayed for possible valid disk boot devices which I can't get to work
Then I just enter on an empty line which drops me into KDB

mountroot>

panic: mountroot: unable to (re-)mount root.