A 'filter' could be written into the online seat booking process.
All it would need, when someone clicked to reserve an overwing exit seat, would be a pop-up window (flashing with a fanfare to attract attention if required!) saying something like:
You have chosen to reserve a seat adjacent to an overwing emergency exit. The exit is a confined space and can weigh in excess of 20kg. If the cabin crew consider you are not capable of operating the door and exiting in the event of an emergency you will be asked to sit elsewhere in the cabin. DO YOU WISH TO PROCEED WITH THE RESERVATION OR CHOOSE ANOTHER SEAT?
A few lines of code, that's all.
