pref64 is the generic term for a "NAT64 prefix".

The pull request helps advertise the NAT64 prefix via router advertisements (while DNS64 uses, well, DNS to advertise it).

The DNSSEC problem is known and has a solution: any stub resolver performing DNSSEC validation should also perform the DNS64 synthesis, should the network be running DNS64/NAT64. The support of DNS64 in stub resolvers is almost nonexistent though (as is the existence of DNSSEC-validating stub resolvers).