Hi, doing on mobile so short answer. To my knowledge they don't do RFC 2146 but rather base everything around a good api that they have. Like you say different types of records etc.

And really, dynip came to be from fortinet/fortigate that have excellent support via their genericDDNS setup and things keep of of grew from there to what you see today.

And the subnet ipv6 sounds really interesting. Will need to check that out, sounds like that could be a feature request