I finally received a pair of U5 adapters and they work with the
Quadstick in USB modes 1 (Dual Shock 3 mode), 4 (Boot in PS4 USB mode),
and 7 (Dual Shock 4 mode).
I'd recommend setting Boot in PS4 USB mode and turning on the Titan 2 flag, like Drew shows in the video:
Turning on the Titan Two flag prevents the Quadstick from sensing the U5 as a PS4 console and trying to continuously "authenticate" with it.
I have not yet tested the touch pad or motion sensors, but boot in PS4 USB mode should allow those outputs to work.
I also used a
short USB extension cable between the PS5 and the U5 to avoid excessive torque on the USB connector of the U5 with all those heavy cables hanging off of it.
For the power source for the U5, I used an
OTG adapter to power the U5 from the USB-C port on the front of the PS5.