So in order to use a globally-reachable IPv6 address you should use a
port from a provider network in the instance.

Thanks Brian for the prompt response.

Does this mean that the only functional IPv6 scenario in Neutron/OVN is
where VMs are directly connected to an IPv6 GUA provider network?

I ran into a similar question a while back: http://lists.openstack.org/pipermail/openstack-discuss/2020-June/015682.html
But there I was wondering / discussing if requesting a prefix larger than /64 was possible to allow using that routed prefix to host i.e. a VPN solution.

Prefix delegation of GUA prefixes implemented in Neutron apparently: https://blueprints.launchpad.net/neutron/+spec/ipv6-prefix-delegation
And to me prefix delegation of a global unicast address prefix (GUA) seems like the cleanest solution.

Regards

Christian