The University here is OK with providing the worldwide mirror, however they are less than pleased that on-campus CentOS clients are not using the local mirror by default.
This isn't a perfect solution but it does work rather nicely ...
If the on-campus CentOS clients use dns resolvers that you control, you could add a zone for mirrorlist.centos.org and direct it at an http server that you run. Then whip up a script that takes (via GET) release, arch, and repo and have it return your mirror.
Thanks, Jef