you say I'm wrong yet you describe the same system as I did. Just because someone slaps Camelbak on the bag doesn't mean it defies physics.
Let throw jr high science class out the window here and visually think this out. Yes the bladder is on the bottom, yes there is LESS air in the bladder then the hose forcing the water up through the tube. So picture this, the standard camelbak is on your back, full of 100oz. The hose is route back up through the pack, over your shoulders and bite valve is hanging roughly at belly level. Right? The the valve is lower then the top of the bag pressurizing the entire system and water is in a closed system at the disposal of the bite valve as the only exit. The first time used, you might suck on air for a second or two to get the pressure going in the positive direction. After that the water is right at the valve for the remainder of the 100oz. Now you ask why hasn't the system lost pressure as less water is in the bladder, well there is still no air in the bladder so the system is flowing in the positive direction.
Hip bags work the same way, but the hose doesn't go up your back, is stays wrapped around the bladder to keep the positive pressure. You will be sucking on air more this way if keeping the valve at shoulder level. Drop the hose/valve down to hip and burp the valve to bring water back to the tip.
I'm darn sure the cubic volume of air in the empty hose calculated to have less volume of cubic air in your mouth. Reason why one good suck from the bike valve will get you water.
If you extend the hose to say a pannier pack, you now added cubic volume of air to the hose. And more volume of air that your mouth holds. Meaning it will take a few hard sucks on the valve to even TRY to get water but your fighting gravity and physics here
Sure you can drop the bite valve lower to get higher pressure for an instant fix but over few minutes the pressure will fix itself and putting water in half the hose between the bladder and the bite valve.
Other then simple physics, I would NOT want a dangling hose anywhere near my rims/spokes
