in cold cold weather i have to make choices... you have to also think from closes to the body out.....

i try to keep my food/energy bars and stuff for the day close... its so much easier to eat em when there not frozen , normaly a small amount of water in my coat pockets to help stablize the temp and keep it from freezing , normaly a nalgen ... i also keep spare batteries and electronics i need including flash lights inside my inner layer.... any thing extra that can take the cold i push to the out side pockets , and then fabrics and stuff not really effected by the cold in my pack....

another trick i been working on is using the camel back safe electrolye solution , this should add enough salts to lower the freezing temp but also act as an anti freeze in the lines... i would try to dry the filter as much as possible... i dont see a problem with charcoal filter media but i dont know about ceramics while wet , they should be ok.... i normaly keep those in my pockets sence its normaly small and i try to dry out the system as much as i can... as long as you dont leave standing water in the lines the freezing should be fine and will go away when you run your filter... but the other issue is what plastics you use for your tubes... vynil gets really brittle when its cold and it breaks down fairly quickly ... tygon has worked well for me in the past....

at night below 40 i find a warm bottle of water is good in a sleeping bag ... so are warmed rocks ... most the time what i try to do is maintain loft in my bags and pad enough to keep the insulation high enough....

i havnt been to a winter camp in 4 years though , most the time i just spend the day and most a night out with out making camp and go home late.... i must be getting old