The hardware store has devices that help with the water hammer situation, like a little reservoir of air that you attach to the system to act as a shock absorber.
I think you're going to find out that the water flows through the radiators in series. The water recirculates from the tank, through the radiators, and back through the tank, right? You would then have to bypass any particular radiator instead of cutting off water flow.
What about controlling the flow of air through the radiators? You may want to use fans anyway.