This drove me bonkers. You can enable play music (etc) in Android Auto for workspace accounts by enabling apps in Gemini. From memory (looking at the settings now, not 100% sure of the magic steps required), but go to admin.google.com, go to 'generative ai', 'gemini app', and 'apps settings', then turn on 'other Google apps'. This lets you play music (and other things) in Android Auto.

Ah, that could be it. I saw that "Other Google apps" and didn't think that would mean Spotify, but I guess its Android Auto or Google Assistant stuff. I'll give that a try, thanks for the tip.