In my opinion, if OR is smart enough to make a Top Mesh, it should be smart enough to make a Terrain Element directly from a Corridor.
I believe, that if you enable top mesh display and create the Terrain, you can then switch back to a display that is Components Only and somehow, the terrain is still linked to the top mesh.
I believe, that if you enable top mesh display and create the Terrain, you can then switch back to a display that is Components Only and somehow, the terrain is still linked to the top mesh.