Simple. Have a client add-on and server add-on.
Client add-on is simply an overlay that is VERY black (perhaps solid black) around the 'light' and the 'light' is just transparent, perhaps with a whitish tinge.
Server add-on, when enabled, forces players to use said overlay.
Server is hosted on a light map, the overlay provides client-sided darkness/light.
If, for whatever reason, you wish to disable your 'flashlight', instead of disabling the overlay, it replaces it with one that is filled in the with the dark color.
Would this work?