I have not found any means to keep a nipple system from freezing here in the north, without compromising the safety of the coop. None whatsoever. Once those temps dip below 0°F and your nostrils freeze shut instantly upon going outside, there's pretty much nothing you can do. Kick the rubber bucket or get a heated dog dish.
It's been a rough winter. My nipple waterer is out of commission, waiting for spring.