Pounder
Phaethon was here
Weasel Op said:since this is for custom maps only, couldn't you just reverse LM and regular plains, making plains "Canal" and LM "Plains"?
That would work. That sounds like the best idea so far.

Even the map wasn't 100% custom, if you initially generated a random map for your scenario with the intent of adding things like canals. You could make a visual scan of the map and any unwanted canal walls could be cleared simply by marking them landmark.
There will still be an issue with the land mark grabbing some tiles from the regular terrain (which is now canal), if the canal is persistant then simply change that coast tile to:grass, desert, floodplain or tundra.