The append-only mode can be implemented using https://github.com/restic/rest-server https://github.com/restic/rest-server or services like rsync.net that offer

The append-only mode can be implemented using https://github.com/restic/rest-server https://github.com/restic/rest-server or services like rsync.net that offer read-only zfs snapshots. Doesn’t solve the asymmetric crypto of course.