seems inconsistent to me.
with the screen on, I get the voice dialer thing (not google voice search, the old school voice dialer)
with screen off, i get nothing. I tried bluetooth launch from the play store and didn't get it working there either.
I also tried this on my Nexus 5 with very similar results....although, I was able to launch google voice search for the first couple times, then it just stopped working.
FWIW, using an LG Tone+ headset and searching for ideas as well.
would love voice search to pop up with the screen off.
Nexus 7 (2013)
Moto G (VZW)