I'm running my own personal koji instance building packages for use in our company servers (Based on RHEL 5 and CentOS 5 currently). Currently the fedpkg "sources" only supports fetching source tarbals from the fedora servers as it is hard coded into the pyfedpkg module.

I have a patch (attached) that partially implements the ability to iterate several servers for packages. (so I can maintain building packages out of pkgs.fedoraproject.org and my personal git repository)

Ideally this should pull out of a conf.d style configuration directory and walk several urls for fetching. As for uploading, that would probably need some extra command line args to support different upload servers as well.
Since it's really just me building packages, the uploading is not as vital, as I just have a simple script that runs a monitors (via inotify) an ftp upload directory and "moves" files uploaded there into the correct location after upload.