The Dakine packs where the hydro tube runs through the shoulder strap tends to do quite well at insulating over those which are fully exposed. Even when I used to use the most basic helipack with no insulation many years ago, that would never freeze the actual reservoir, just the exposed tubing.
In general, I'd say that most of the time they won't freeze, as if you are using it and you can also blow air back into it, then it keeps it a little warmer. The times mine froze were when it was below zero and I was out for extended periods of time.
A bit like platypus though, these days I only ride with a backpack (I use a Dakine Heli Pro DLX) when I'm going off-piste or taking a camera up to shoot something. I don't need a bag on my back when riding around the resort or in the park. I can fit a bottle of water in a side pocket (baggy pants are great for that) then I can fill it up again later and probably grab something at lunch anyway.
Obviously it depends on the person, but I never feel I need to be carrying layers to switch, as I've always been fine and have been able to vent using zips.
