Kopia allows you to run scripts before and after a backup. It also allows you to redirect the base from where to read the files.

They Document it for the likes of FreeBSD, for Linux one would skip the mount step and just set the snapshot path to the magic .zfs directory.

https://kopia.io/docs/advanced/actions/#zfs-point-in-time-sn...

Why wouldn't you use the magic .zfs directory on FreeBSD as well? Are the instructions assuming an older version?