This feature will be added in the next release.
There is a small issue to be aware of: Motion Lighting doesn't usually turn lights on from motion if they are already on. This new Scene for off feature can trip over that if the Scene activated leaves the controlled lights on. So, there is also a new option added for the next release called "Do turn on if already on". So if the Scene activated leaves the controlled lights on, this option has to be selected, otherwise ML would not turn them on the next time there is motion. This option is mutually exclusive with the option "Don't turn on if turned off manually". If you happen to need both options, you're out of luck for now.