A True Android Infotainment system

That's a pretty cool idea. I have never heard of anyone doing this, but from a development point of view it sounds plausible.

 

I have often thought about this too-- in addition to the nav functions there are many other uses that would be great in the car (streaming Pandora/SiriusXM/etc, checking gas prices on Gas Buddy, real time speed trap alerts on Trapster, traffic avoidance routing with Waze... The possibilities are endless!)

My thoughts are that it would make more sense for an aftermarket company to produce a head unit that runs Android natively, and either uses Bluetooth to share your phone's data connection, or partner with a cellular carrier to offer a low cost data subscription (~$20/month.)

As far as your questions, HDMI will not pass touch controls, only video and audio. Pioneer does make a head unit that connects to Android phones and mirrors some some apps on screen (called AppRadio,) although it only works with a handful of phones and a handful of apps.

Perhaps, if you have a compatible Motorola phone, you could modify a lap dock to mold the screen into your dash... I don't know if the lap dock is touch screen though (since it has a mouse and keyboard.)

 

There is currently a bounty on XDA to give people full control of their phone while using the Appradio2. It shouldn't be overly difficult to do as the iphone guys are already reporting success on their end. Just need to entice a dev to tackle it.

[BOUNTY] Appradio App MOD for Pioneer Appradio 2 $675 SO FAR!!! - xda-developers