Actually, to make rust, you need AIR. Water alone will not make rust. An example would be deep sea wrecks that are wooden and are still there centuries later. Even wrecks in the great lakes are very well preserved because the water is very cold and deep with low oxygen contents.

I'm assuming that your radiator elevation rises (when hot) and falls (when cold). It's this headspace where the falling coolant can trap air into the coolant which gets left in the block. The air/water combo is what's attacking the bare metal. I'm wondering if the "catch-can" is to low and allowing an air pocket to develop. And now that you mention it, I'm wondering about my own car. I'm not worried as the cast iron will be along time before a catastrophic failure, but still.

Great topic Dave! Now my mind is curious!