How will this be implemented? I think that this is the ACME v2 spec (though there's no link to it in this blog post or the ACME v2 announcement). The only reference to wildcard domains is
Wildcard domain names (with "*" as the first label) MUST NOT be included in authorization objects
and there's nothing in particular in the dns-01 section explaining how to answer a DNS challenge for a wildcard domain.