I have never used preseeding so I can't help you on that.
However I have tested boot time hooks on a number of occasions. Up to version 3.0~40a if I remember correctly.
I would use it like this:
Add your adduser hook prefixed by a number higher than 118 (but do not use 999) and add it to config/includes.chroot/lib/live/config/
Something similar to this:
$ cat > 120-adduser << EOF
echo "live-config hook: adduser"
adduser user group
It is not necessary to specify any boot parameter.