Tested with prototype PCB
Made a simple PCB layout with all of the same key positions as the original Pi 400 keyboard. The matrix keymap is correct.
As long as you use the available keys in the matrix you can make a replacement keyboard connected to the Holtek controller.
Connections to the Pi 400 PCB were made with 30awg wires. I got these on Amazon, it is a roll of 8 different colors bundled together.
The keyboard end with labeled pads.
The Pi 400 matrix pads. I labeled the pads on some masking tape.
The 8 different wire colors in 3 sets, total of 24 connections.
The completed ORTHOPI is here.