This is a combination of something I did in the past combined with a light weight idea.
On a couple of occassions my wife and I have had two days of hiking to a water source (not uncommon in a dry year in the Southwest). On our first occassion I had a heavy Jansport pack with a cap that made a detachable fanny pack. I had filled a mylar box wine bag with 6L of water (they hold more than 5 when not in a box) but found the extra weight made my pack extremely uncomfortable. After a while I rigged up straps and carried it on my chest. My total load was quite heavy but I could do it all day without the fit of the pack being uncomfortable. It recently occured to me that the two water bottle holders on the front straps of my ULA Catalyst are no only convienent but shift a little weight forward. With a couple of straps added to an already carried stuff sack and the mylar bag which weights very little, I believe it would be a way to easily carry extra water and more weight than most of the light packs can carry comfortably on your back.
