I just thought of this idea for my star Trek scenario. It's probably not original but it was a great revelation to me. I've tested it out and it works.
In the events.txt, create a unit every turn (i.e. turn = -1) on a spot that is the wrong terrain (like ships on land). Nothing will appear. Use an event to change the spot to the right terrain and the tap is on. Use a second event to turn the tap off by changing it back to the wrong terrain again.
In the events.txt, create a unit every turn (i.e. turn = -1) on a spot that is the wrong terrain (like ships on land). Nothing will appear. Use an event to change the spot to the right terrain and the tap is on. Use a second event to turn the tap off by changing it back to the wrong terrain again.