Since the default plane in Unity have just a Mesh collider, you can replace it with a BoxCollider and make it very thin. I guess a BoxCollider performs better than two Meshcollider with a 200 tris mesh ;) (the plane have 10*10*2 tris).
The opposite Raycast was also my first thought, but I guess you will detect a lot of other colliders than the one you'd expected. I think I would use a BoxColider, but it depends on your needs.
