@mjg59
Some of the pro-grade small switch boxes let you arbitrarily tell at least one side, possibly both, what capabilities they should announce (and also do other useful things like pull audio out to spdif and strip hdcp). They might do what you need — see the HDFury line. Kramer also make an HDMI firewall (for when you need to make sure an untrusted tv doesn't get access to a sensitive network via HDMI, I suppose), but it's as pricey as one might expect.

@dymaxion @mjg59 is there actually any known device which has implemented Ethernet over HDMI or is that an entirely theoretical threat? Any time I’ve looked I’ve been unable to find anything (which is mildly annoying, I’d love to use my TV as an Ethernet switch for all the things plugged in via HDMI).