So, this seems to be a pretty amazing service. I also see that it has an unofficial API, for google music, which makes me wonder:
I use Google Play all the time -- just stream it from my phone via Bluetooth. Given that the car doesn't run Android, it would be a significant porting job from both Tesla and Google to get it in the car. Since they didn't even go that route for navigation, which IMHO is far more useful to have full Google integration rather than just getting the map tiles from Google, I wouldn't hold my breath.
It's essentially just another mobile device running an ARM processor, so it's definitely in the realm of possibilities, but I wouldn't count on it anytime soon.
All this may be possible with native android emulation support! This Android Developer will keep his fingers crossed!
Android isn't a requirement for google play music. I use it perfectly everyday with third party apps for both Windows and Windows Phone. Each app is from a different developer. The API is open for any dev to build it's own app for, just as the Spotify API. I think the only difference from using the native apps is that you need the payed subscription to use a third party app.