Bluetooth Speaker microphones: Can they be used for "Hey Siri"?
To corroborate, my BT speaker-with-mic (a FuGoo) and BT-capable car (a 2012 Mazda) will will route microphone input and speaker output to Siri, but only if she's summoned by button press (long press of the play/pause button on the speaker, long-press of the iPhone's home button in the car). Neither will work with "Hey Siri". I suspect that one obstacle is that neither device has its microphone continuously listening - they are only activated when needed (a call comes in or you trigger Siri manually).

I suspect if a BT speaker manufacturer could make "Hey Siri" work, they'd be sure to list it in their list of advertising bullet points.