Spoggy, I don't think your question is getting answered. I would speculate that if you are circulating the water and have a heating source that is warm enough, you shouldn't have any problem with keeping the water from freezing. I'm not sure how to calculate the total amount of water that would be in that much pipe plus your reservoir (you could probably figure that out pretty quick by capping and filling a length and then finding the volume). I would just make sure that whatever your heat source is would be able to handle that amount of water.
If you didn't circulate the water, you would have the added problem of more surface area around smaller amounts of water (when its in the pvc pipe) and loss more heat transfer (loss) from the water. I think if you circulated it, you would somewhat, but not completely eliminate that problem. So as I'm thinking about it, you would probably need a heater rated for higher quantities of water than what you actually have. Maybe something that has 50% more capacity to be on the safe side.
I've heard others on here say good and bad things about using aquarium heaters. Some I think have recommended something heavier duty and meant for outdoors like a birdbath heater or something.
To answer your original questions (to the best of my ability), I think you could stick with the nipple waterers in the winter, as long as you can keep the water warm. If my understanding of thermodynamics is correct, as long as the water stays somewhat warm and is in contact with some part of the nipples, the heat should transfer easily through the metal and keep them from freezing. Where I'm foggy is on the availability of a heater that could do that (although I'm sure you could find something). Circulating would definitely seem to help.
Best of luck and let us know how it goes.