Previously:
Might be simpler in the long run to make it a 2-step process. That is, do a minimal OS install, then have a 2nd DVD or USB key that they install after the base system with a script that upgrades any existing rpms and installs the rest of what you need.
That adds an interesting set of questions. Do I somehow try to get the postinstall script to prompt for it? Do I try to create something similar to firstboot that would prompt for the second disc after the reboot? Do I just list it in the install docs? Right now the install docs are "1) Boot from DVD; 2) Choose system from menu." I'd like to keep it that simple.
Seems like I'd still have to create an appropriate comps.xml file for whatever was on the second disc anyway, so why not just create everything on the first/only disc to begin with? Also, I always copy all of the RPMs into a local repository (e.g. /opt/repos/centos6u4) and run createrepo on it. Having one comps.xml file that works for both the installation disc and the installed repository keeps things clean...
Regards, Doug