I had the idea when I needed a TLS certificate for a system without a global DNS record. I recalled that http://xip.io/ exists for IPv4, but didn't find something similar for IPv6. So I had to do it myself :-)
Thanks for the feedback, I somehow missed that mistake. As a quick fix the server now returns NOERROR for both unknown records and names. That's still not 100% correct, but better than nothing.