That makes very much sense.
OTOH this will confuse people. `Want to have Legacy updates? Then go get "updates" and optionally you may want to use "legacy", but the latter is not needed. But if you don't use "legacy" you will be unsupported by the Fedora Legacy team. But don't worry, you can still have "updates". Hey, why are you running away?'
Maybe fedora.us should become Fedora Updates instead of Fedora Extras and Fedora Legacy? Or maybe all together?
Confused? Once again? ;)
Anyway I's also vote for a fedoralegacy.org repo not policy inflicted by "Repository non-mixing" statements. Matthias' layout seems very reasonable and can be setup in a matter of a few minutes.
Try to keep legacy independent.
fedoralegacy.org has no need to warn against interoperability, as long as you avoid the "legacy" channel. "updates" can be mirrored in many different places that previously published updates, including download.fedora.us and freshrpms trees. We all win.
Warren