Wasn't "dual-stack hosts talking over a v4 network" what you wanted?
v4 routers don't understand and can't route v6 packets, so you can't use the same route for both of them. v6 packets have to go to a router that understands v6, and v4 packets have to go to a router that understands v4.
I have no idea what "or a superset in the later stages (shared down to /32, v6-only in deeper subnets)." is trying to say.
It's perfectly possible to turn v6 on without changing anything at all about your networking. When e.g. Windows Vista came out, hundreds of millions of people did exactly that.
Why can't the service just set up NAT64 to accept your v6 connections on their v4 servers?
v4 routers don't understand and can't route v6 packets, so you can't use the same route for both of them. v6 packets have to go to a router that understands v6, and v4 packets have to go to a router that understands v4.
I have no idea what "or a superset in the later stages (shared down to /32, v6-only in deeper subnets)." is trying to say.
It's perfectly possible to turn v6 on without changing anything at all about your networking. When e.g. Windows Vista came out, hundreds of millions of people did exactly that.
Why can't the service just set up NAT64 to accept your v6 connections on their v4 servers?