I think the live user gets only created at boot time. Seehttp://live.debian.net/manual-2.x/html/live-manual.en.html#287 and
http://live.debian.net/gitweb/?p=live-config.git;a=blob;f=scripts/config/0030-user-setup;h=d0939a23a87d2b17e9992ffa5b66c441760d8917;hb=HEAD.
Note that /etc/skel is involved, too.