I bought a Camelpack to try this in my TK armor. Removing and installing the bladder in the back plate would be easy enough, but the problem was running the hose in the helmet. The way they work, it's almost impossible to do. The only practical way would be to have the hose slung over your shoulder, remove your helmet to take a drink.
But historically, whenever we troop, the only time we remove our helmets is away from public view like in the back room, etc. where we generally already have water waiting.