For the most part, your indoor relative humidity levels won't be too different from outdoors unless you have added moisture to the interior air. (hot steamy shower, etc) So generally speakling, having your rad outside should not be that big of a risk as the colder it is outside, the less the air can hold moisture. For any one in a cold climate, have you ever noticed how much more chapstick you use in the winter months? Now if your computer room is right next to your shower, you may want to take additional steps to protect your system. (like keep the door closed)
Also, running 15% Zerex antifreeze combined with the heat sources from each block and the motion of the water should keep the water from freezing. If your worried enough about it, make some various concentration mixtures of water and anti-freeze, put each mix into a container and put them all in your freezer. Check back in 24 hours and see which ones have froze and which ones haven't. That should tell you exactly how much anti-freeze you'll need.