Reveal information hidden in GM layer with a reversed Fog of War type of function

This idea stems from the concept of having hidden elements of the map that can be uploaded as a partially transparent file that contains for example the names of towns, landmarks, traps and secret doors... this information image would overlay perfectly with the map/background and avoid a thousand individual files being uploaded for every bit of information.  It could be the inverse of the fog of war function. Every item hidden in the GM layer that falls in the selected area becomes visible to the players?!

Am I misunderstanding?  When I purchased Princes of the Apocalypse, the are maps were full of information on the gm layer that could easily be bumped to the map layer when they were discovered by the players.  Is that not what you're suggesting?

Yes, but not all published modules have neatly cut out elements. Some modules just don't have that and some of us want to make their own maps. Cutting out elements ourselves is very long, tedious and... it's a mess to add everything one by one. On the opposite, adding a transparent layer containing all the information you wish, that is already sized to overlay perfectly with the background, in one single upload, would be very convenient.  Also, instead of revealing elements one by one from the GM layer, just selecting an area that lits up the player screen would be very neat.