The Google Play Services beta program hasn't done anything on my phones yet. They had a notation that it could take a few hours, but nothing has happened after two days for me. The phone status shows the beta program active, but perhaps there is something I need to do to kick off an update. I've sent a query back to Google for help.
Google came through with the beta update this morning. I Updated to the November 1 version 14.5.73, and tested behavior while navigating with Android Auto. It performed for me precisely as Numbernine described, working flawlessly. I am very happy with the results.
Bugs... The volume control on HU or steering wheel changes the currently playing audio. If navigation plays a message and you adjust the volume at the time, you would only affect the navigation control. BTW, not sure about Google maps, but in general you are not allowed to discuss any issues with beta testing outside of the beta forums.
I don't see anything about NDA or such on the page so don't see why it would be an issue.. its a public beta so I doubt they care, especially if it fixes issues.
I've diagnosed the issue and found a workaround if you don't want to mess with betas et al. The issue is triggered when you get a notification while plugged in to android auto. Work around is to put your android phone on "do not disturb". You won't get notification sounds obviously, but the music controls will work as expected.