Thanks for the bugreport to Thomas Lehmann:

If I change the USERNAME in /etc/grml/grml-live.conf from "grml" to "user" (or
anything else) then I am not able to use virtual consoles 4,5,6 from a newly
build custom live CD.
This is because the user "grml" is hard wired in /etc/inittab within the grml-
live build process.
Maybe the script /etc/grml/fai/config/scripts/GRMLBASE/21-usersetup could be
the place to adjust the user name according to the custom setting?