I am led to believe that the sensorcore can use detectors (gyro, microphones, magnetometer etc) without powering them. Don't ask me how! Therefore, the always-on listening will not have a huge effect on battery life. However, quite why this is not true for the S400, too, is unclear to me; perhaps it just does the same but with less sensors - and does not include the mic; but that is a wild guess.
In any event, none of this seems to me to exclude the option of a software solution that allows 'Hey Cortana' whilst the phone is on charge. It strikes me that in situations when I would require this feature, that would be the case - whilst driving; when the phone is across the room; on the bedside table, and so on. At all other times, the phone would be in my hand or next to me, and it is easy to use the search button.
Just a thought.