The keyboard matrix driver is on the main board, and there’s no access to the USB2 bus that connects the matrix driver to the USB controller. So you would have to run the whole Raspberry Pi in USB “gadget mode”, so it pretends to be a device rather than a host. The USB-C power port can also support USB-2 OTG in gadget mode (but it has no high bandwidth features).
That would be kinda cool, not sure if it could be hacked to do that as I know it has USB host ports and if you could present them to another device as a client port - I just don't know.
If you could - be the ultimate keylogging keyboard.
Me - I'm still trying to find out if the power button is a hard power cut-off or a soft one which will allow a graceful shutdown.
"Power off is achieved by holding down Fn+F10 for two seconds. This is a soft control that negotiates with Linux to shut down, so you don’t corrupt your memory card or your USB drive. Power can be restored by pressing F10 (or Fn+F10)."
Thank you thank you thank you. That is good to hear, making the Pi 400 a way better option for so many for just that feature alone as standard.