Thanks @chrishamm. I now have a RPI400 working as the SBC with a Duet3 6HC, using an unchanged DuetPi image. It will be controlling a RatRig KillerBee CNC. I'm using a SunFounder 10.1" touchscreen, and the RPI400 is powered through the touchscreen, using only the screen's 12V PSU. I have version 1.01a of the Duet3 6HC, so its 5V is not used to supply the RPI400. So far I have tested a limit switch, and a stepper motor, using DWC, and all looks good.

Hi @hlwerschner, did you have any success connecting a RPI400 to a Duet3 mainboard?