Yeah, your thread reminded me that I've been wanting this too. I found two ways:
Reassign Your Bluetooth "Phone" Button to Open Google Now
Now, with screen off I can long press bluetooth button and get voice search prompt.
(when using the Bluetooth Launch app, you'll most likely need a magnifying glass in order to see the options. the font is tiny.)
After testing this a few times I notice that the screen does light up but won't turn off - at all. No time-out. This makes it unsatisfactory. I did some more searching and found the app Hint2Search
https://play.google.com/store/apps/details?id=dynalogix.hint2search
Have been testing this one on my M8 and it seems to work great. The screen does come on but will auto-dismiss just like normal. Too bad it won't do its thing fully behind a blank screen. But with a quick time-out, there's less chance of a mistake happening in your pocket.
Also, it seems to me that Tasker and the AutoVoice plug-in shoud be able to do this. I've emailed the AutoVoice dev and asked.