for quick edits of debian live

mount -t proc none /mnt/chroot/proc
mount -o bind /dev /mnt/chroot/dev

requires

sudo apt-get install squashfs-tools
sudo apt-get install genisoimage
sudo apt-get install qemu qemu-kvm libvirt-bin

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sudo unsquashfs filesystem.squashfs
rm filesystem.squashfs
sudo mksquashfs squashfs-root/ filesystem.squashfs -e boot
cp filesystem.squashfs into_your_live_directory_from_the_iso

general outline of the process

mount the iso to a directory
copy the squashfs to a writeable directory
unsquash, chroot to the directory made by unsquash
edit /etc/skel to your preferences
mksquash
copy to writeable iso directory
remake iso
test