I use an MSR Hydromedary when I climb in the winter.
After filling the bladder (with boiling water), I add some chic-drink (eg Gu2O, Cytomax, etc) - the increased salt concentration also delays freezing in addition to the other hydration benefits. The sack gets shoved down into the pack's pad pocket and buried in gobs of insulation in your pack, the drinking tube comes up through your hydration tube hole (I use a McHale w/guide harness so I hacked a hole above the shoulder strap and reinforced it with a rubber gasket). The tube goes down the strap as normal (you may need to buy a longer Tygon tube if your bladder sits low in a bigger pack, stock tubes are usually too short for this). Tube is insulated with REFLECTIX (not neoprene, reflectix is way lighter), which is that bubble wrap insulating stuff you buy at Home D. One layer of Reflectix, then some thin duct tape strips to keep it in place.
Then you home sew a TUBE SOCK shelled with silnylon and lined with fleece, with an inside diameter large enough to slide over the reflectix-insulated tube. Both ends are open. You slide the fleecy tube sock over the tube, tuck in one open end into the rubber hydration hole gasket (it stays in place fine). Make sure it's long enough to go past the bite valve by at least 3 or 4 inches. You know the eskimos that wear the huge parka hoods with 8" front face tunnels fringed in fur? that's the effect you're working on here. You could even fringe the end of your tube (Google Fur Trim but don't come crying to me if your p0rn flags get overloaded ). And use one of those camelbak winter bite valve caps as well. It helps.
So then, when you need to drink (you can do this 1-handed), slide the fleecy tube sock up the tube, pop the bite valve insulating cap off, suck away, blow back into the bladder, pop the cap back on, and pull the tube sock back down.
This rig works remarkably well down to well below zero, assuming you are actually drinking your water and not letting it fester in your bladder all day. it should keep water for you for 6 hours, by which time you should have probably burned thru 2 or 3L of water anyway assuming you're moving up on a climb etc.
And, in good lightweight style, consider the dual use nature of your new fleecy tube (hopefully with fur trim) :o