I'm off to Seattle Fabrics to see what I can find for buckles and I'm planning to get enough hardware and webbing to make it a sternum strap replacement, particularly for the 2 liter version. I'm hoping the 2 liter stuff sack will handle a 1 liter Platypus PlusBottle without being too floppy or in the way. I concerned that 2 liters of water (in the 4 liter stuff sack) may be a bit much. We'll see.
I have also been looking for UL waist packs to carry water as an accessory to SUL packs without pockets. It is all too easy to end up with a waist pack that is heavier than the pack itself, which is a 1-step-forward-2-steps back endeavor. But a waist pack would place the heaviest item (water) on an SUL list on the hips where it belongs. I am imagining a rig with a broad fabric "saddle" and a buckle system like the Golite Peak (vee's of small webbing and a small buckle) and simple bag pockets to hold a 1 liter bladder on each side. That should allow building a waist pack in the 4oz range. I can see making a no-sew version by coupling two 2-liter Sea-to-Summit bags with some webbing, but the waist band would be narrow and probably slippery. Making a sling bag this way would be very easy.
Update:
I didn't find buckles that were compatible with the Sea-to-Summit and the store I went to didn't have 2 liter bags. I sacrificed another bag to get the buckles and fastened them to my shoulder straps with 5/8" webbing and ladder buckles. I did look at some of the newer Outdoor Research lightweight dry bags and noted that they have larger buckles, which may make it easier to match up off the shelf buckles. Once you have the right buckles, you can work with any of the dry bags from a particular manufacturer and change sizes at a whim.
Anyway, I tried it with a 1 liter dry bag and the 1 liter Platypus PlusBottle. That seemed pretty good and didn't feel like a lot of weight or hamper. Then I tried the 4 liter dry bag with a 2 liter Platypus and that is workable too. The larger bag feels a little more ponderous, but it doesn't bounce much. I walked up and down some stairs and it bounced more on the way down, but those are pretty short choppy steps. I it is less ungainly with the 2 liter bag at 1.5 liters, etc. The weight of the bag negates any need for a sternum strap.
The 1 liter combo:

And the 2 liter Platypus in a 4 liter dry bag:

Details:
