But some sort of pool of "additional" <s>public</s> globally unique
prefixes that can be requested, bound and routed to an instance would be
nice - kinda like
https://www.terraform.io/docs/providers/openstack/r/compute_floatingip_v2.html#example-usage
.

We made a conscious decision to not support floating IPv6 in the base reference implementation, although it is available in one of the third-party drivers (Midonet?).  Making the tenant IPv6 subnets reachable directly was instead the goal, which happens when PD is used.

I suppose you are talking about this fip64 feature - https://docs.midonet.org/docs/latest-en/operations-guide/content/fip64.html // https://docs.openstack.org/networking-midonet/latest/user/features.html ?

Just in case there might be a misunderstanding in what I described ... "floating ip" for IPv4 would translate to "floating prefix" (out of a pool, with a limit on the number of those a tenant might use/request) in my suggested case and the routing sending the traffic of that prefix to an interface.
Certainly no single ipv6 addresses / 128 shall be "floating" around or be mapped to a fixed IPv4 to make things even more complicated.

It's just a about being able to provide an instance with a global prefix of its own to do networky things.

What would be the process of getting such a feature into a working
specification in case one was interested in implementing this?

It's outlined in the neutron docs, but basically it starts with a bug describing the feature you'd like to add, it's then discussed at a weekly meeting each Friday.

https://docs.openstack.org/neutron/ussuri/contributor/policies/blueprints.html

-Brian

Thanks for that immediate pointer.
I shall be discussing this with some folks here and might get back to writing one of those blueprints.

Regards

Christian