The rDNS is a DNS zone delegation (unlike IP addresses) and I have full control of the zone. There is no problem with making non-ip.subdomain.9.3.4.4.c.1.7.0.a.2.ip6.arpa exist.
As a consequence of the delegation system, ip6.arpa proof of control are also a proof of control of a IP subnet
Seems I am not the first to do that https://crt.sh/?id=32094798