What's wrong with Hydrapack? Is this some rumor you heard or first hand experience? Not sure what kind of reservoir you're talking about, but my Hydrapack I use for mountain biking has a plastic clip you slide over the top of the entire thing. I don't see how that is ever going to pop loose. I have taken many falls on my mountain bike which are arguably harder hits than snowboarding falls and I haven't had any issues. I think it is a fantastic pack and prefer it over the numerous Camelback packs I have used.
I have a hydrapack in my dakine helipack too and it works fine. I am not really worried about water leakage either as I've taken some spills on the bike and it's been fine. The only thing I can see is the possibility of the hose freezing up out on the slopes. I know my camelback water bottle does, causing ice to form in the mouth piece.