The correct way to set up BackupPC to run full disk backups against a remote system is to create a backuppc user on that system and then add an entry to your sudoers file for the most restrictive possible backuppc command. Here's an example from one of my servers:

    backuppc ALL=(root) NOPASSWD: /usr/bin/rsync --server --sender ?*

This primarily is to help limit the amount of damage that backuppc can do if something very strange happens and it decides to try to eat the filesystem.