Just had a thought for those trying to use hydration systems in winter and dealing with all the different tricks (blowing water back down, insulating, tucking in jacket, etc.) to keep the tube/bite valves from freezing. Since most bladders these days have a detachable hose, why not bring two hoses? If after all the other tricks fail, swap out the hose and put the frozen one in pocket near your body to thaw out, then repeat if necessary. Again, just a thought, I haven't tried this yet.
Also, people often say that once the tube freezes, the system is useless, but you should be able to still drink water from the opening you used to fill the badder. I have done this on camelbak bladders - it's not ideal and requires two hands, but the system isn't completely "useless" if the tube freezes.
Personally, I always bring a back up bottle either full or empty.
