So the idea would be to have something like msync-v6.centos.org::CentOS-v6, and ask you to provide us the ipv6 address for your mirror (we can already do that in advance if you advertise AAAA record on your side for your mirror) and so it would be an "opt-in" thing (at least as a start)
The separate DNS ensures that people will only get what they ask for (v4 or v6), but why have a separate module name? Put both v4 and v6 acls on the CentOS module and clients will get the one matching the DNS name they use.
Scott.