I have two units that have funplug installed and I use an "at" job to backup another server to each of the DNS units.
The script uses rsync; I can see no reason why this couldn't easily be achieved based on my working setup.
I also, at this stage, manually backup remote servers for clients with screen sessions and using rsync; the remote sites are standard server machines that have ssh running. I use key authentication and to get keys working automagically, I use ssh-agent and ssh-add tools on the DNS units.
Edit: Oh and to lessen the CPU load on the DNS units, I use the "arcfour" cipher.
Here is a sample rsync command:
/ffp/bin/rsync -avxH -e "ssh -l -o -c arcfour" root@other_server:/source-backup-location/ dest-backup-location
An odd thing about using "date" on the DNS box is that it presents the GMT date/time, not the local date/time, but the at scheduling uses actual localized timing.