If this is RHEL5, why not just add a repository in your ks file that has all the updates in it? Far easier than trying to reconstruct the install tree.I don't know why folk are giving EL5/Fedora answers when it's clear OP isn't using one of those. See where he says "I've tried linking the 'RedHat/base'."
Because upgrading to something more current has a few advantages. Not just updates, but being able to install your own RPMs at kickstart time too.
Of course, nothing is stopping %post or firstboot stuff for older distros. --Michael