In that case, the fix is simple - inside extrafiles.tar.gz there's a text file with all the files to be in the new chroot. add it to the file, recompress the .gz and you should be good to go
I added the file I suggested (the way you described it), and ... backupmenu is working again. So there is no reason not to push a new version in the repo with "cp /lib/libpopt.so.0 /tmp/disk/lib/" added in filelist.txt.