I'll be running only the N7 (non-3G) as the media player in my car. It will auto-tether via wifi or BT (undecided) to my Samsung Galaxy S3 when I'm in the car.
I've looked at AUTOREMOTE (which can be run as a module for TASKER as the main app) to send a push msg to the N7 when my phone receives an incoming call (or starts to dial out). This push msg goes to the N7 where I use tasker to pause the music for the duration of the call (and resume playing once the phone is back on the hook). This works pretty well (during benchtop testing) with a Wifi + fast internet connection (eg: ADSL2+/4G/LTE) and is able to pause the music on the N7 in under 2 seconds. Trouble is, when I'm in the car, I'm only on a 3G network (and sometimes in a carpark basement with no reception), so this notification takes 7+ seconds to arrive, defeating the purpose - I might as well pause the N7 manually.
I've started a thread with the Autoremote dev about a direct device to device connection (via Wifi/BT, that works without internet connectivity) but it's not currently on his priority list (not enough demand).
If you're interested in the ability to directly communicate between android devices (ie: push notifications) without Wifi/BT, please show your support in my thread (linked above). This kind of feature could be used for SO much more than just this application!
Also, if anyone knows of any other methods for syncing a pause/mute between their phone and the N7 (via Wifi/BT) I'm happy to hear it.