I did not get an answer on my question, if the Palette 3 will support UART over USB. (my question was a bit longer) So my assumption is: You can use the Palette 3 with a Duet in such that the Palette 3 can Stream the G-Code to the Duet - like you would do with Marlin. How ever, then you loose the luxurity of the RepRapFirmware Interface for those prints. A Stream of the control commands from RRF to The Palette 3 seems not possible. Be aware this is just my assumption based on combination of informations i have, but maybe this combination will help you. :)