I fail to see enough gain to program, test, and support a feature that technologically makes no difference what so ever. However, I'll ask anyway .. What's wrong with creating an ipv6.* roundrobin of AAAA records?
The technology already works and supported, ipv4 clients can't use AAAA records so it will be ipv6 only.
Lets also remember that since services doesn't accept incoming user connections, they can only REACT to what has already happened, not RESTRICT something that is in the process of happening.