Ok. Between the moment of submitting this topic, and this reply. I had a burst of inspiration to not fixate on the 127.0.0.1 aspect of things, but use the external network IP as the listening address…
AND behold! It works! Even from the host’s-host that is running the Hyper-V hypervisor (or that special container that has all access). So for anyone in the future finding this, might I suggest not using 127.0.0.1 as a listening address.
Can someone enlighten me why localhost wouldn’t work for me? I am curious.