I've not done this....but I've seen a couple people on a trail I ride use a big Camelbak but strap it to the rear rack and have a really long hose that rests on the top tube so they can easily reach down and grab it while riding.

So they get to carry a large volume of water and put all the weight on the bike frame instead of their back. They don't get the giant sweat patch on their back from wearing a Camelbak. And it's still easy to drink without stopping.

Mounting the bladder on a rack is easy. I have no idea where you get a really long water hose though. REI maybe?