Thought I'd post this as it took me some time to work out it was possible.
External control surfaces now work with the Audio Track Mixer, but I couldn't find a way (or any info) on how to make it work with the new Audio Clip Mixer. Eventually I found 'Toggle Control Surface Mixer Mode' in the keyboard shortcuts and now everything is working well in both modes.
This works for mute, solo and level in the Clip Mixer. I've not yet attempted any automation, I can't see a way to turn on the 'record keyframes' other than manually one track at a time. Still, progress to this point is very welcome.