Thanks - the repo name and hierarchy are useful to understand.

I didn't want to mirror a whole repository, just those RPMs I needed for my builds.

It's ugly, but what I did was run livecd-creator with the outside repos until it had finished downloading the headers into each of the directories
/var/tmp/livecd-creator/<temp-dir>/yum-cache/[lcdr_c6|lcdr_e6|lcdr_lcd6]/headers

Then use these headers to get the names (replace .hdr with .rpm) of the RPMs needed, and download them using wget.  Then run livecd-creator again, but with the local repository.  For subsequent builds I did the same thing but had a script that only downloaded RPMs I didn't already have.

Each time you add RPMs to your repository you need to run "createrepo path-to-repository-folder".  See
http://fedoranews.org/contributors/richard_flude/repo/

Luke