nmap_mass_rdns in nmap_dns.cc has this little disappointment:

  // mass_dns only supports IPv4.
  if (o.mass_dns && o.af() == AF_INET)
    nmap_mass_rdns_core(targets, num_targets);
  else
    nmap_system_rdns_core(targets, num_targets);

I don't see any technical reason why IPv6 couldn't also be supported, which would give a big speed boost to IPv6 scans, especially -sL list scans, which can be a cheap, stealthy way of doing target discovery.