I know people who use hydration systems outside their jackets- they've developed two solutions. A, get a slim line carrier and wear it under your jacket, or B, get some reusable heat packs and toss one into the carrier (wrapped in a sock).
And you've just discovered why I carry firestraws, birch bark, fat wood slivers, cat tail fuzz, untreated cotton balls, little candles, esbit tabs (powder them) AND tinderquicks. :P
_________________________
-IronRaven
When a man dare not speak without malice for fear of giving insult, that is when truth starts to die. Truth is the truest freedom.