My mother has a small goldfish pond and in the winter she uses a small floating water heater to keep it from icing over. I am sure you could use that to keep things at a fairly standard temp. Here is an example at only $12
http://www.aquaticeco.com/index.cfm/fuseaction/product.detail/iid/9056/cid/2218 Maybe that will help keep you where you want to be temp wise. If you want to make sure the line does not freeze up, the warmer temp due to the heater and allowing for a slow drip ( crack open the hand valve ) will provide flow and should keep the line from freezing over. Where there is flow you generally dont have a freeze.
