Most of the Bluetooth modules I worked with have programmable in/out data lines. So no fancy tricks with LEDs or photoresistors would be needed. The complexity would lie in getting it to trigger at exactly the right moments.

Related (but not exactly the same thing), I have recently tested that, with my current setup, the following thing works:

- iPhone is connected to Honda factory stereo using its "Apple Car Play" feature.
- iPhone is running Waze with voice directions turned on.
- BlueGigaEmpeg is connected to Honda stereo as a Music Device.
- BlueGigaEmpeg is playing music.
- Waze plays a voice direction: Music turns down briefly and voice direction plays atop it.

The above only works if the phone is connected via Apple Car Play, though. It doesn't work if the phone is connected via Bluetooth. I don't think the Bluetooth spec has a command specifically for voice-direction-interruption (though I could be wrong about that).