I would to give a normal (no sudo) user the privileges to backup (but not modify) system files like /etc and so on. Deja dup haults on this as it says no permissions. Is there a way to create a group of users say 'backupers' that can do this, then add the desired user to it?
Maybe then one could edit visudo, adding a line that allowed the user group to do this without password? Something like the following?
# Users in the backupers group are allowed to run back as root.
%backupers ALL=(root) NOPASSWD:/usr/bin/cp