The salt will precipitate and block the radiator fins
Of course you know you can dissolve things like salt into water. You probably also know there's a limit to how much it can hold - called saturation. Saturation is dependent on temperature.
If you saturate warm water and then let it get cold, the material will precipitate - in salt's case, crystallize on a solid surface. Where will it do this? Where it is the coldest, and since your use-case calls for temperatures below 0C/32F, that will be in the radiator, inside the fine passages.
This is a perfect recipe for clogging the radiator.
But more worrisome, with the salt not in the water, it is more vulnerable to freezing. The colder the weather, the poorer this "antifreeze" performs.
Also, it will corrode everything.
As discussed in Paulster2's answer.
You can protect the engine with zinc plates, and you already are - the radiator has a lot of zinc. The radiator will sacrifice itself to save the engine, so good news there anyway.