So in short, you want a chasm? That won't bode well with the current method of making water though...accidentally make the water too deep and you got a bunch of black chasms on your map. Also, you said:
the game would know that no units can traverse that terrain.
Air units? I would assume they could just fly right over it, same as normal.
Edit: Oh, and what about the reverse? Mountains a size higher than the current max height, which air units cannot pass over?