Here's some notes from our internal Wiki for enabling BackupPC. With this method, there's no reason that you couldn't backup the entire system.
Enable ssh access
System Preferences/Sharing
Check - Remote Login
Create and populate /var/root/.ssh/authorized_keys
Applications/Utilities/Terminal
sudo su -
mkdir .ssh
chmod 700 .ssh
echo '[backuppc user public key]' >> .ssh/authorized_keys
chmod 600 .ssh/authorized_keys
exit
Note the "backup public key looks something like:
ssh-rsa AAAAB3NzaC1yc... [a couple lines of hash] ....ISXXYosqZQ== backuppc@server