John,
I have SIDI shoes with Campy cleats also. My shoes came with a filler piece for the cavity. I would go to your LBS and see if they could hook you up. Without the filler I would think the cleat could slip easily riding on the rails.
I ride Look and Campy for the road. Shimano MTB style for dirt, commuting, and coffee rides. MTB shoes for easier walking.
Keith