Sorry to revive an old thread, but this still has not been addressed.
https://bugs.freenas.org/issues/2375#ticket
It appears it has been discussed to death by the devs a year ago but they couldn't come to agreement about where the geli metadata backups should be stored.
I already posted to the ticket about a month back inquiring about it again. Specifically, I asked "Why not just make them available for download in the interface like geli recovery key?" and have not heard back.
Even if i went and submitted a patch to correct it but allowing for web UI download, I'm not sure it would be accepted, because the devs seem to believe this requires some greater level of security or convenience when backing it up or something (note the mention of cloud backup tie-ins in the bug report by one developer).
Maybe someone here has some inroads with the devs and can ask, but I really don't know what else to do.
It might help if more people went on that thread and inquired about it to let the devs know you want this added.