@dc42
Okay I got it to work. I was powering off the Duet after erasing and resetting the panel, which gives the white screen state. In this state, it does not seem to work.
What did work.
With the Duet powered on and plugged into the panel
hold the erase button on the panel for a few seconds
press the reset button on the panel
DO NOT power cycle the Duet
The panel will remain on, displaying as usual
Bossa will now recognize the panel via it's serial port
Upload the new panel firmware as usual via Bossa
When complete, unplug the usb from the panel and power cycle the printer.