Those get populated based on what you set as the LIVEUSER name in the /etc/remastersys.conf file. If you want to bypass that, just edit /usr/bin/remastersys and comment out the section that creates the casper.conf file.
Remastersys is just a bash script at the moment so you just have to put a # at the beginning of the lines you want to comment out.
Coment out the following lines:
cat > /etc/casper.conf <<FOO
# This file should go in /etc/casper.conf
# Supported variables are:
# USERNAME, USERFULLNAME, HOST, BUILD_SYSTEM
export USERNAME="$LIVEUSER"
export USERFULLNAME="Live session user"
export HOST="$LIVEUSER"
export BUILD_SYSTEM="Ubuntu"
FOO
cp /etc/casper.conf $WORKDIR/dummysys/etc/
to be like so:
# cat > /etc/casper.conf <<FOO
# This file should go in /etc/casper.conf
# Supported variables are:
# USERNAME, USERFULLNAME, HOST, BUILD_SYSTEM
#export USERNAME="$LIVEUSER"
#export USERFULLNAME="Live session user"
#export HOST="$LIVEUSER"
#export BUILD_SYSTEM="Ubuntu"
#FOO
cp /etc/casper.conf $WORKDIR/dummysys/etc/
Edit - uncommented the copy of the casper.conf as that still has to happen.