Even with blowback and an insulated tube, I had freezing problems. Then a valve broke on me once and soaked me. I switched to carrying a small Platypus bottle in my chest pocket and it's worked out great. I top it off whenever I go in for a pee break.
My 1/2 liter Platypus finally croaked and I tried the liter today in a chest pocket and it was better than expected for it's size. Another convenience of the water bottle is at some areas you can simply reload at hydration stations on the hill. Currently, looking for a 1/2 liter option more like an MSR bladder, versus the plastic Platypus. Any options out there?
Regarding the, freezing issue, under the jacket works best overall and my Osprey Switch pack has a leaner, removable bladder pack. With less water is usable under a bulky shell. Plus less to catch on chairs and trees. Another option is to sew in a hole and flap in a jacket for the tube from an external pack to route to the inside and around the front.