2 gallons a day sounds about right. I've heard others mention they use 2 gal a day in Moab for MTBing.(all day, not just on the bike)
2gal=7.3 liters, 256 ounces, 16.5 pounds, 7.3 kilograms
It is easy to see how high you can suck, just try it. Height is all that matters at that low flow rate. I think it is very do-able, I've seen under seat hydro units for sale. try just the bladder in a light homemade polyester(its UV blocking) case. (similer to a pillow case)
Try a platypus, they are great hydro bags and hoses, I like them much better than some other more catchy designs I've tried. (Like the large/widemouth screw on opening of the nalgen bladders, hard to fill with liquids and the seal sucks unless you get it just right)
I have a 3L big zip and a 2L hoser/platy-resevoir(same thing minus the hose) The "big zip takes a bit of muscle to pinch closed, but it doesn't slosh out while you try to close it(because the bag is upright, not on a slope) and it doesn't leak at all. I prefer to unzip it only for solids and cleaning, and normally just use the small hose/cap end for water fillup. The platy reservoir only has the small cap opening and quite frankly does a fine job. The hose can swap between all of the Platypus bladders/bags/bottles, that normally have plain caps, and the bite valve is super high flow. I have run out of water rather quickly because, at first, I didn't realize just how high flow the bite valve was.
Everything from 12ounce to 6 liter(210oz)
Originally Posted by Platypus
Platypus® Bottles, Reservoirs, Water Tanks™, and Hosers™ all use the same threaded spout, allowing you to create your own system to fit with your individual needs.
Combine a 6L Water Tank with a Shower Kit or a 1L Platy Bottle with a Drink Tube for a small-capacity hands-free system.
I know all that sounds like a plug but I like the product. Note DON'T CLEAN/STERILIZE WITH IODINE. I let mine soak with a basic iodine dish sterilizer(after foolishly leaving juice remains in it for days) and the plastic strongly attracted the iodine. It then slowly released into my water giving it a horrible taste for ages(finally found a cleaner to remove most of it).