HOLTEK HT45R0072
The keyboard of the Raspberry Pi 400 is connected to a dedicated MCU that uses one of the 4 ports of the built in USB hub.
D+ and D- USB data lines are circled. They go through the vias to the other side of the board and run across to the VLI USB chip. The large test pad is VBUS.
On the other side of the board the D+ and D- lines connect to the VLI chip.
On the bottom of the PCB under keyboard ribbon connector the matrix pins are broken out. If you could reprogram the Holtek controller you could connect a custom keyboard matrix. Unfortunately this is a OTP (one time programmable) device, so unless you can work with the existing matrix the Holtek is not very useful.
The easiest way to replace the keyboard would be to cut the traces between the USB hub and the Holtek chip. Then connect the D+ and D- to a separate controller.
The keyboard has a power key (Fn+F10). This functions similarly to the modification I did on the Flirc case. A custom replacement keyboard could use an io pin to trigger the power on/off the same way.
