Foreground app logger to the rescue (possibly)!
Using the app logger I watched what happened when the window comes up and get:
com.android.systemui
com.android.volume.SecVolumePanel$SafetyWarning
I'm guessing I can use this as a foreground app check after (attempting to) setting volume, UI interact to press OK, then set volume for real.
I think this warning only comes up once after a restart.