How do you add a preseed file to the initrd?

[Tim Edwards <timothy.edwards.ext@siemens.com>]

I'm trying to add my preseed.cfg file into the root of the initrd, as 
mentioned is possible in the debian preseed documentation: 
http://d-i.alioth.debian.org/manual/en.i386/apbs02.html#preseed-loading

So once I've mounted the boot cd, copied the initrd.gz to a temp 
location I try to add it to the cpio initrd file:
cat preseed.cfg | cpio -ovcA -O initrd
cpio: premature end of file

If I try to fully extract the cpio file it tries to create device nodes, 
which it can't because I'm not root (and don't want to become because I 
need to package all these steps up into a script that won't run as root).

So how is this supposed to be done? Surely someone here's had to do it 
before?

Tim