With casper, I can put a "casper-sn.squashfs" file at the root of the USB flash drive and the content will be overlaid to the root.
I've read the great documentation at http://live.debian.net/manual/stable/html/live-manual.en.html as well as some related man pages but I see no reference of using squashfs persistence image files. Is this feature available in live-boot? Thanks, Ka-Hei