I have an automatic watering system, so all of my rabbits have to deal with valves. Transitioning from a bottle to a valve has never been a problem - they might bite at the valve if they don't get water out right away, but they pretty quickly figure out how to get the water out. The climate here is mild enough that I seldom have to deal with the system freezing for more than a few hours; when it does, I use bowls/crocks (and have some rabbits that dump them as fast as I can fill them).
The ball and tube type of bottle does release water more easily, but it works on a vacuum, so it can't be adapted for the heated waterers that fill from the top (plus, the balls are notorious for sticking in the tube, not allowing any water flow at times).
If I had a rabbit that I thought couldn't figure out my water system, I would put a bottle on the cage for a few minutes while I was there to observe. If the rabbit drank eagerly from the bottle, that would indicate to me that it hadn't "gotten" the valve yet; I would do this several times a day to make sure the rabbit didn't get dehydrated. Sooner or later, the rabbit will simply ignore the bottle because it has been getting its water needs met by drinking from the valve.
Since your rabbits have a long history of using a bowl, i suppose you could give them a bowl a few times a day, and keep an eye on the water level on the heated waterer. Since the valves don't drip (unless wear or some foreign matter gets in the way of forming the proper seal), any drop in the water level should indicate that your rabbits are drinking from the waterer. Getting sufficient water is critical during the winter, since rabbits need water to process their food. If they can't drink, they won't eat, and food is the fuel that keeps their bodies warm.
I don't like the idea of putting anything in the valve to try to attract them, particularly anything sweet. We are talking about an environment loaded with bacteria; giving them food and lots of nooks and crannies to grow in sounds like asking for trouble to me. You can take the valve apart to clean it out, of course, but there are small bits that are easy to lose; and finding replacements isn't always easy.
