Vodka
I don't think I would add salt. Especially if you are feeding pellets. I, personally, have never liked water bottles for a couple of reasons. Many of them leak, and will rust out the cage wire prematurely. Most all options have their disadvantages in cold winter areas. I like crocks, for several reasons, but regular crocks can freeze solid and crack overnight if it gets really cold. Plastic crocks get chewed and the abrasions harbor places for germs to reside, plus, in an emergency, one cant heat them on a stove or with a torch to thaw them. I finally went to aluminum water crocks. I tried a few at first, thinking the water in them would freeze much more quickly than in the stoneware ones. I was please to find that the water only freezes a little bit faster.
No mater what one uses, unless it is a "piped" system, the best solution is to have double containers. I finally bit the bullet and bought double aluminum crocks. It worked best for me. They will never break, and I can put the frozen ones on top of my wood furnace in the basement and they will be thawed for the next morning (or night). Of course I have been raising rabbits for a while, and figure I will be doing it for another 20 years.
Maggie, uses the inexpensive plastic refrigerator containers. She can change them easily, and if one gets chewed, etc. it is not much of a cost. I thinks she ties one in a corner and places a similar one inside which she alternates as they freeze. I may not have it quite right, but I know she has her method posted somewhere on the forum. If I was just staring in rabbits, or wasn't certain I would be keeping the for more than a few years, I would probably go with her method. It think it would be the easiest and most cost effective solution.
If you really like the water bottles, or if your cage is too small for a crock, you might simply double up on your bottles. IF you have several rabbits, as I do, a 5 gallon bucket makes it easier to carry the thawed/frozen waterers back and forth to the house.