I did the following:
sudo nano /mnt/mmcblk0p1/cmdline.txt
I entered
home=mmcblk0p2 opt=mmcblk0p2
I removed home and opt from /opt/.filetool.lst and ran a backup:
filetool.sh -b
The problem is, when my RPi reboots:
1. There's a fresh .filetool.lst with only two entries: home and opt
2. The previously created cmdline.txt doesn't exist anymore.
Once again, I must have forgotten something I guess ...