Yeah ,I have thought of that as I have started working with flcs... I guess it very clever and makes room for endless variations of this idea/trick/possibility.
If only the palette is correct...
General 666... just keep the Transparent color slots correct
An actual part of the Map Graphics could be used for the Flcs or IF you used a Blank Flc, All Flcs could use one frame and only have Magenta in them. Set the Magenta as the Transparent color and then you can name the "Unit", make a .ini file, add all to the Unit Folder and add the "unit" to your game as you would any other Unit in the Editor, Civilopedia Image files and Text Files.
You could pre-place this invisible "Unit" on your Map where wanted. IF set as Immobile it will stay put.
Problems would be that Units have a Health Bar and that would show. Additional problem would be if it is desired as a thing that would maintain throughout the game, it would have to be set where it would be unlikely Killed... Mile High Health Bar
IF desired as a Short lived Thing, that would be the way to go with such a "unit" so it would be ok when it is destroyed but generally it would be best to set terrain as impassable rather than try to use a "unit" to prevent it.
An invisible "Force" Unit could use this technique and be pre-placed on the Map. Then IF an visible animation is desired for any part of it, that could be added such as the Death or a visible attack, etc... many possibilities. A Ghost or strange "power" that is part of a Haunted Mountain, etc...

An Invisible "Force Field" would be Good for this...and when destroyed, that could represent the Forcefield being destroyed or turned off rather than a unit Death.