I have put my medium sized camelback in my spacepack, and have trooped with it under the armor.
the armor doesnt fit right with the CB in there, but we were hiking up several thousand feet, so it was necessary for safety.
I dont think you can get one in a cannon pack though. the boxes might be okay, if you cut into them from the frame side, but you'd have to get tube extenders form a sports store or somethign to get the hose to reach under your armor and up to your helmet.
My question is. Do you do events where you have your helmet on that long, or rather in a troop where you do not have the ability to go take a drink in a "out of sight" location?