The Livebox CRAMFS Filesystem used on the earlier firmwares can be extracted from the CRAMFS image BUT requires root access as you will be creating dev nodes.
I used these cramfs tools which contains the sources to both cramfsck and mkcramfs and built them by just running make with defaults. These appear to create Livebox compatible files.
The command to extract the files from the image is cramfsck -v -x directory_to_extract_to cramfs_image_to_extract_from. This will create a directory containing all the extracted files which you can then edit.
To create a new CRAMFS image I used mkcramfs directory_to_create_from cramfs_image_to_create. This will create a new image containing all the files from the directory specified.
I have used the commands within RedBoot to copy the image to the Livebox and then flash over the existing CRAMFS Image, although you can do the flashing using the programs contained in the running firmware as long as you have command line access. Both methods are more difficult with the latest firmwares.