...Just a regular EEPROM? This dongle is detected as "DVI" (same as the HDMI monitor I have).

I also discovered that most if not all DP-to-HDMI plugs are also detected as "DVI". I tested 3, and all three are detected as DVI. Even my hacked dongle does that.

Below are the images of the dongle, with the black solder resist scraped off for visibility.

(Image flipped to make reverse-engineering easier)

According to my multimeter, magenta (not pink) is connected to the black pin (middle pin of the three-in-a-row side of the chip).

Pin 1: Main Link Lane 0 + (Shorted to ground, magenta)

Pin 6: Main Link Lane 1 - (Shorted to ground)

Pin 12: Main Link Lane 3 - (Shorted to ground)

Pin 13: CFG1 (yellow)

Pin 14: CFG2 (blue)

Pin 17: AUX - (green)

Pin 18: Hotplug Detect (red)

Pin 19: Return (cyan)

This is a problem because if the card is thinking this is a DVI or HDMI plug, that also means it cannot be used to manipulate DisplayPort. This is probably a dead end. Very disappointing.

However... Remember the weird behavior with the logic analyzer? I discovered that by simply supplying 3.3V to the HPD pin, the graphics card thinks it is still connected to the monitor, as long as there WAS valid EDID information supplied before it shut off. Maybe it's time to pursue this road?