I've thought about using a waist hydration pack with a small UL pack that doesn't have side pockets for water bottles. Water is a big part of the weight on a day pack or XUL kit and you recoup some space while you are at it.
But the waist pack can add as much weight as some light packs and they aren't cheap. Contrasting the REI Double Shot waist pack with the REI Flash 18:
Double shot: 14oz, $34.50 (normal retail, on sale for $20 at the moment)
Flash 18: 10oz, $29.50
It doesn't add up for me. A lot of the weight and cost is in trying to stabilize two water bottles for runners. You don't need all the compression straps and padded holsters for walking. The rest of the Double Shot design departs from typical UL pack design principles with heavy fabric, zippers, etc, etc, etc. The design is fairly typical of waist hydration packs.
It wouldn't be too hard to work up a XUL belt that would handle a couple water bottles and it would be hard to resist the urge to work in a pocket. With a small backpack that is carried high, you could wear it with the bottles to the back, but I would want the option of carrying it either way. There are a lot of waist packs that have the buckle on one side of the pack rather than dead center, and that would allow carrying one in the front with a pack that is long or has a waist belt-- the fuss is about having a fat buckle in the middle of your back. My hunch is that you would need a fairly wide fabric band to stabilize the bottles and provide enough traction and hip-hugging to keep it all from sliding down: you would have around five pounds with two liters of water, bladders/bottles and waist belt.
The other option that came to mind is to carry one bottle or bladder on a shoulder strap, bandoleer style, outside your pack shoulder straps-- not much different than a bota bag. You could make a rectangular bag in silnylon with a shoulder strap to hold a one liter Platypus, which would just be a rectangular bota bag when done.
When comparing waist packs and bottles, keep in mind that the bottles are typically smaller than a liter. For example, the REI Double Shot carries 41oz (1.2L) total.