Is there a way to gather the list of all domains/subdomains, etc? Because if they are servers, it is unlikely there isn't a dns entry against each of these ipv6.
Most nameservers disallow zone transers (requesting the full zone file), preventing this. Additionally it's more and more common for IPs to have a fixed generic PTR that just describes the IP instead of the host, and for a lot of hosts to not even have dns records. (My last webapp never bothered with dns for the dozens of ephemeral webserver instances).