Sigh. All these advances, all this technology. And what have we gained? Expensive, hard-to-clean "hydration bladders" when somehow, for eons, people managed with a goatskin, a steel canteen, a Nalgene bottle...
Dont get me wrong. I use hydration bladders, and find that in a lot of situations (especially bicycling) theyre great. But on a hike? Puh-leeze. Part of the pleasure of the trip is saying, OK, when we hit 4,000 feet, were gonna stop for a water break." And then you do. Stop. Packs off. Bottles out. Ten minutes to drink, rest, socialize, look around. This nonsense of hike, hike, hike, hike with a tube in your mouth is for the birds, in my humble opinion.
But, to your question. Is there a bladder that can sit in a handy spot in the pack? Ummmm... no. Bladders have to be oriented vertically, so water goes to the bottom via gravity, enters the tube, and then transits to you. A bladder sitting on top of the load likely would cease to send fluids even though its still half full. It just wouldnt have the proper orientation. They simply have to be near, or touching, the bottom of the pack.
The fix is easy. Carry a bottle too. Attach a bottle holder to your belt or pack, such as Outdoor Research's Water Bottle Parka ($24), that keeps cold drinks cold for hours. Then relax. Hike a little, stop to drink, hike a bit more, stop again. Its great! Trust me.