Dumping the remote server to a local box that's in a NATted network.
From the local box, open a reverse tunnel:
$ ssh -R2222:localhost:22 remoteserver
Now you're on the remote box, so continue typing:
$ sudo -i # /sbin/dump -0uan -f - / | gzip -2 | ssh -c blowfish -o \ UserKnownHostsFile=/dev/null -o StrictHostKeyChecking=no -p2222 username@localhost dd \ of=/datadirectory/server-`hostname`-full-backup-`date '+%d-%B-%Y'`.dump.gz