I did this with an animated GIF. It puts a specific pattern on the display that maximizes the EMI generated by the HDMI cable. You can hear the morse code on an RTL-SDR by tuning to ~300MHz or so.

The animated pattern in the GIF file is practically invisible (only 1 LSB color change). You can set the GIF as a tiled background for your Web site and detect if someone nearby is visiting it that way.

You could use it exfiltrate data that way, it's enough bandwidth for a password or crypto key. A fairly short piece of JavaScript is all that's required to do this. If you keep repeating the message periodically, you can vastly expand the range of the signal, by averaging it at the receiver.

This works with any device viewing the gif via an HDMI cable? Or only if user is browsing via Raspberry Pi?

It works with any device, as long as it's using TMDS over HDMI. I don't think it will work with DisplayPort as it uses data whitening.

I've been trying to dig up the code for it. It's somewhere on my system.

Update: Here it is: https://pastebin.com/rwtTvFRQ

Interesting bit is that the Raspberry Pi 4 leaks at 125 MHz without a cable connected. Guessing that mostly a manufacturing defect on some boards and not universal to all RP4.

They may be measuring the RGMII between the SoC and PHY instead of the 100BASE-T1.

The cable was connected in the test.

Not in the demo video published in the author's blog: https://lipkowski.com/etherify3/

Holy crap! I stand corrected. That's quite a waist of energy to have that ethernet block powered up all the time.