Use car headunit without android auto

I'm thinking it may be possible to make an app that mirrors what is shown on the phone screen to the car headunit by emulating the Android Auto communication protocol. Touches on the headunit screen could be transmitted back to the phone for interaction with the apps. Most of what Android Auto does is screen display over USB and some touchscreen interaction with the user so it may not be hard to get some of the features.

Does this already exist? If not, do we know if anyone is working on this?