That would require support on both HDMI ports (it's a negotiated protocol, not something that can just be passively wired), and I've never heard of any mass-mar

That would require support on both HDMI ports (it's a negotiated protocol, not something that can just be passively wired), and I've never heard of any mass-market hardware that actually supports Ethernet over HDMI, despite actively searching for it on multiple occasions. Discrete graphics cards on PC don't even support CEC, which would actually be useful for some people.