Hmm, initial test caused the usb hub to lock up, ignoring any connects or disconnects, but I just flashed the Duet from the Pi without USB power from the Pi. Could be the CH340 driver from the other device I tried with that caused the issue. Ymmw.
Worth noting that it seems "port 2" is connceted to another USB hub that powers all 4 usb ports on the 3B+ as only -P 2 has any effect, and it affects all 4 ports at the same time.