Here is the procedure for creating a custom Archlinux Live CD (or USB), by modifying an existing Live version.
The simplified version is: copy an ISO, make its (read-only) content editable, chroot into that system, edit it. It your case you edit the /usr/share/X11/xkb/symbols/us file if you want to edit your US keyboard. Leave chroot, create a new ISO from that system.
See the Archlinux wiki for the details. I suppose for Debian-based distros it's more or less the same.....
Another idea: openSuse Build Service, can be used to make a live CD - have no personal experience with this, just a thought.