Duck on Wheels just got a packable hydration vest from:

http://www.nathansports.com/

You might be able to get your stuff in one of their models without the water bladder full. She tried nearly every running hydration pack in REI. All of them chafed under the arms except this. Says because they adjust vertically, horizontally, over, under or on the bQQbs and so far is comfy in all her training for the half marathon.