I have a 7" Android Tablet mounted in the car connected to an amp via standard ISO connectors which I made up myself. This setup allows me touse GPS, media etc... exactly as you describe. both the phone and the car have TabletTalk which works well as far as connectivity, but the lack of audio was frustrating.
It seemed that one needed a seperate Bluetooth device to utilise handsfree, but then it wouldn't come through the car speakers, and it wouldn't mute the media.
I have found a possible solution though. STRIKE iK-1 BLUETOOTH HANDSFREE CAR ADAPTER KIT.
Apparently this thing connects to a head unit (in our case tablet/amplifier set-up) to do exactly what we're talking about. The drawback is that any phone must be paired to both the tablet AND the Strike ik-1 for this to work, and the tablet and phone must also use TabletTalk. But it seems it would provide exactly what we're after.
Thoughts anyone before I go shelling out my 'hard-earned' to experiment???