As i have already mentioned it should be possible to show the screen content via http using my app (https://github.com/nerone-github/WebViewAuto) (or any other webview app to be exact).
Basically we need something which can pass user input (tap) from Car -> Phone, and the Image from Phone -> Car.
Now at least i can confirm that the second - Image Phone -> Car is not a big deal. It even works without root using this wonderful application
Now all we need is the input. So anyone got an idea how to do that?
I know that this is basically "against" the Android Auto concept of providing a distraction-free UI. But ultimately this will probably never be an easy thing to do, and probably just used by experienced and hopefully responsible drivers.