In reference to the limitations: Then I suggest to just hook the button for selecting the CAR screen. I feel this can be possible, and we use this event to trigger some intent. Please, think that sending one intent is by far more simple that printing on the screen. So, perhaps the first goal can be hook the AA UI for sending events to other Apps. You agree?
Just out of curiosity, what intent did you have in mind? Or what is your use-case going to be? I'm trying to think of something I would want to do on the phone without any feedback on the headunit while driving and I can't think of anything.