O.k. I have tested this myself.
I have no problem spawning characters on the terrain.Provided you place the trigger zone at the same height as the character,which should be placed above the terrain in the editor,otherwise it will spawn beneath it and fall.
Enemies do have collision with the terrain but tend to run on the spot,this can be cured by placing floor segments under the terrain and I find this works perfectly with characters using the terrain mesh for collision.
The only problem you may find is that the terrain surface inherits the material index of the segment floor so you should use a segment with material index 0.
The terrains only use one segment space which is located in the corner.