I've generated my own list of ordered sources. Following are the techniques, scripts, and data. Comments or corrections on any of these would be most appreciated.
Sources of Data
1)
ftp.redhat.com all *.src.rpm files under redhat/linux/enterprise/6*/en/os/SRPMS
2)
vault.centos.org all *.src.rpm files under 6.*/{os,updates,fasttrack}
If anyone feels I'm starting from the wrong data, please say so. I debated about including the fasttrack dirs, but as this point I've convinced myself that it is probably correct, or at worst harmless).
I wrote a script to do all the heavy lifting. For args it expects files containing lists of paths to source rpms (there are too many srpms involved to pass them directly on the command line). The script identifies srpms with identical contents by comparing the list of files and their hashs (so a centos rebuilt srpm with no changes is considered a duplicate of the rh one even though some of the headers change (e.g. vendor, buildtime).
The script sorts first by package name, then by version-release *with dist tag removed*, then by rh vs centos, then by full version-release. An srpm is considered a centos rpm if either the vendor is centos, or a centos dist tag appears in the release, otherwise it is considered an rh srpm.