I get 2 public ips from the cloud provider - one is ipv4 and one ipv6. I have a NetBSD 9.1 host on which I'll setup NetBSD 9.1 qemu guest and would like the guest to get the ipv6 public ip. Is this feasible? Any tips (both NetBSD and qemu side)?
That qemu on a cloud instance sounds like bad design. As Bob said, KISS and do dual-stack. You have one IPv4 and one IPv6, that's perfect to setup a dual-stack MX and outbound MTA.
Besides, to solve the forward-confirmed-reverse-dns issue, I would also KISS over there and use a single and IPREV FQDN for both v4 and v6 IPs, be it delivered as a host from one of the two domains you are targeting, or simply another domain of yours that they would share an MX record from. As a side-note, it's possible to have a CNAME as an MX record, but it's just simpler and safer to point to the MX who's hosting those domain's email, directly.