Circling back to report that this does indeed Just Work, although with one caveat for Windows users. I'm astonished. I tired it in Windows first which because I have an AMD GPU required holding my nose and "upgrading" to 24H2 at minimum. This is (not at all) conveniently the same feature update which rug-pulls WMR capability natively and can't be undone without a reinstall and if you're in the same kind of shoes I am and your WMR headset was working previously, that's probably because you had locked your feature version to that one with third party tools and/or group policy. So that was mildly trepidatious.

But so far so good. My only wrinkle seems to be that the double tap the menu button to activate camera passthrough does not seem to work. (Yes, I enabled the cameras in settings.)