nmap_mass_rdns in nmap_dns.cc has this little disappointment:
// mass_dns only supports IPv4.
if (o.mass_dns && o.af() == AF_INET)
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.
I am working on it https://gitlab.com/g10h4ck/nmap-gsoc2015/commits/hotfix/51
It looks cool !