Let's figure this out. The omni-bag has a zippered opening at the top end, and an attachment for hanging. The bottom would have some sort of quick disconnect system, allowing for attachment of the following:
- Pre-filter and gravity filter for water treatment
- Shower/bidet spray nozzle with shutoff valve
- Stopper for using bag as sink or waterproof storage if shutoff valve fails
Ideally, it would be black to absorb infrared from the sun. One could also boil a liter and pour that in to warm up frigid creek water.
In bidet mode, one would place the full omni-bag on the ground and lean a heavy rock on top to provide spray pressure. The nozzle should be aimable to about 45 degrees above horizontal, so one can squat a couple of feet away and avoid dripping back down on the bag.
To allow for use as a sink, the omni-bag should be free-standing while full with the zipped top open. If this is not possible, it should be usable while hanging with the top open.
You can also seal it full of water, soap, and dirty laundry, then hike with it for a few miles to provide agitation.
What capacity would we go for? 4 liters would provide enough for a quick shower, and also enough to fill up a 2-liter hydration bag plus 2 1L bottles.
Will a hollow tube gravity filter remove trace amounts of soap residue? If not, one would need to exercise care in rinsing the omni-bag between sink and water treatment use.