There are quite a few resources on building a custom Linux boot CD...
is probably my favorite, though it's likely overkill if you just want to make a few modifications.
More likely, you'll want to mount the iso image and extract the squashfs image (most linux installers/livecd's use squashfs, AFAIK). Then you'll need to mount the squashfs image and browse it to do various things (dhcp or static interface config options, add sshd to boot, etc...).
Here's an Ubuntu howto outlining the process:
Hope it helps!