Can I drain one zone?

I think this is an easy one. I have a cast iron boiler, primary-secondary piping, with four zones (each with its own circulator). There are isolation valves on each zone, and a boiler drain valve on the return side of each. One of the zones goes out to a three-season porch which we rarely use anymore. It has cast iron baseboards. The porch will get below freezing if the temps drop into the single digits or lower. I currently leave its thermostat set at 45 deg.

Can I just shutoff this zone, disconnect the circulator power from the controller, and drain it? I am not sure if leaving the cast iron radiators with no water in them is OK. That is, would I get rust/corrosion that would present a problem if/when it was used again in the future? Also, I can drain the return line but that will leave water from the circulator up to the rad on the supply side. Do I need to pull the circulator so I can drain the supply side completely (and then put it back in after draining)?

Any help is appreciated. I'd like to quit heating an unused porch if I can. But I don't want to do something that could cause a problem in the future.

Thanks all!!