That's a good idea. I'd start here:
http://zsync.moria.org.uk/
Yep, client-side rsync. I don't know the patent status in the US
(Google for earlier discussions), and don't want to know. Get it
before the EU becomes a police state too. :-(
It should be an afternoon's work to combine the above with:
http://rpmrebuild.sourceforge.net/
and have a tool that will mirror update directories that contains
.zsync files. Run some tests locally, then convince a repository
in the free world to give it whirl. If it works out, perhaps the
signatures could go in a repository metadata file.