This seems to suggest that while the Studio Display can be driven at 5K by a raw DisplayPort signal (which is great) the audio signal is routed over USB rather than the Displayport audio channel. That is unfortunate, I was hoping to at least be able to get video+audio.
This is correct and indeed unfortunate. Another thing I didn't mention in the post is that the driver for the XDR's USB bus is the same as the one for the Studio Display's ports+camera+audio. I imagine the only way they'd have support "spatial audio" is over USB so they just went that route from day one
I have a LED Cinema Display (DisplayPort model) on Windows that surprisingly responds to Monitorian brightness control when I use current AMD drivers. I used to run a specific brightness control app for Apple displays, but have since ditched it.