Hi David,
as we discussed at Plumbers, I tried some hack to create/delete HDMI/DP PCM stream per hotplug/unplug. Test patches are found in sound-unstable git tree topic/hdmi-dynamic-pcm branch git://git.kernel.org/pub/scm/linux/kernel/git/tiwai/sound-unstable.git
The patches are all small and easy, but it's still in a pretty rough cut. It won't handle some cases, e.g. unplug during suspend, or such, I'm afraid. After all, it's just a test.
With these patches, the PCM device appears and disappears properly upon HDMI/DP hotplug/unplug on my system. On mine, it appears as /dev/snd/pcmC0D8 as it's an Intel on-board. So far, so good.
Now the problem is that PA gets confused when this happens. It can switch to HDMI/DP, but then the analog output disappears from PA's profile. You cannot switch back to analog output after that, even after you unplug HDMI/DP cable.
Or, it might be my PA version... I'll check newer one. But it'd be good if you also check in your side.
thanks,
Takashi