Drawing a line showing the Border is altra-simple; and, won't cause any new problems.
Use the Existing Tile Graphics; and, create a Line on the Tile Graphic.
The Game Maker application allows for that kind of simple Game System Modification.
And, I do know that, more advanced Game Engines like what is being used for Navy Field, uitlizes the very same Graphics Interface Method that Game Maker uses.
Of course, Ultra-Dog is correct...
The Navy Field Creators will probably botch that up by not properly aligning the Border Line from Tile to Tile.