Regarding this, I would like to question the need for such manual configuration. centos.eecs.wsu.edu is your mirror, right? Without any configuration, MirrorBrain would send you all requests from clients out of 134.121.0.0/16 (if there isn't any mirror in the same network of course). If a client is not in that particular network, but within AS10430, it would still get sent to your mirror -- if there is no other mirror in that autonomous system. Would there be a second mirror in your autonomous system? That's the question. If not, everything would happen automatically anyway. No need to juggle lists of network prefixes. (And no need to make such configuration accessible, which could result in a security issue after all, if not handled carefully.)
So far, I didn't encounter a case where clients are outside the network prefix of a mirror, but within the same AS, and there is a second mirror in that AS -- so there was no need to add a way to specify network prefixes at all.
We have two mirrors in our AS, which has several IP ranges split across two datacenters over 2000 miles apart. Being able to specify a CIDR for each mirror would be nice since there's no guarantee an AS lookup alone is going to get it right.. but I suppose as long as BOTH mirrors are returned on a mirror list, the fastest mirror plug in could easily choose the correct mirror.
-Randy