yes, and if yum was able to collect repodata that spans multiple repos and multiple hosts for those repos - what you are saying will work fine. But, I dont think it does.
Sorry, Karanbir and Michael, but you've lost me. Would the updateinfo.xml.gz not be generated on the CentOS repository itself? E.g.,
http://linuxsoft.cern.ch/cern/slc510/updates/x86_64/RPMS/repodata/
http://linuxsoft.cern.ch/cern/slc65/updates/x86_64/RPMS/repodata/
CentOS repos have the other XML files in their repodata directories, but lack the updateinfo.xml.
Shatil (@shatil)