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.)
