hi
My way requires you to install grub2. which may not suit you? but I will explain the theory....
http://sidux.com/index.php?module=Wikula&tag=Grub2slit
Pretending you have some free space on a spare or real partition...create the folder to hold copies of your existing structure...I will call it build/
under build goes all folders for your real system....instead of sub-folders to build/boot....eg build/boot...build/dev etc
I would cut out /usr/share/doc
/boot/grub/grub.cfg will need some modification as described....but your kernel may be already suitable with the initrd to be bootable as is?
genisoimage does not care if its not bootable...it will generate a iso...which you test in a virtual machine.
what will catch you out....is a compiled kernel away from the stock standard...that for example has compiled n against iso9660.
BTW I have not done this myself...but the thought did cross my mind