I found that my machines access URLs that look like the following when they connect to RHN, but the part after the date doesn't seem to be predictable, and i'm not sure what to put for the version string in the middle.
http://xmlrpc.rhn.redhat.com/XMLRPC/$RHN/redhat-linux-i386-9/listPackages/20030924151830 http://xmlrpc.rhn.redhat.com/XMLRPC/$RHN/redhat-linux-i386-7.3/listPackages/20030924165059