I wouldn't suggest using common car antifreeze in copper radiators. It corrodes copper pretty quick if the copper is exposed to air after having been in contact with antifreeze. Bubbles finding their way in and out of your radiator could eat a hole in some of the weak points in a few months.
There are plenty of non-automotive antifreezes out there though, and even some food-safe options. I would recommend one of those so you can work on your loop and eat donuts in a peaceful state of mind.
Just be sure to call the manufacturer and ask if their product has any reaction to copper, nickel, aluminum, acrylic, and anything else that might be in your loop. They never list that stuff on the bottle, they just assume their product is only going to be used for the single thing it was designed for.
Freezing is only an issue if you get below -20c I would think, but rog is calling me a liar. He probably has some pretty good flow rates on his setup. The faster the water is moving, the less of an issue freezing becomes.
Hey rog, didn't mindchill build you a rheostat that controls for dewpoint temperatures? He may have another customer if my memory isn't being dysfunctional about this.