I know at least for California residential water use makes up just 15%[0]. I know every little bit helps, but a toilet is an incredibly simple invention and it would be a shame to replace it with something so much more complicated.
[0] http://www.environment.ucla.edu/media/images/water-fig1-lrg....
https://en.wikipedia.org/wiki/Composting_toilet
Alternatively, just try to divert urine so it never mixes in the first place:
https://en.wikipedia.org/wiki/Urine-diverting_dry_toilets
Both of these seem on first glance to be easier to maintain.
When I see things like "Turn a 2L bottle into a light" for a really poor area - I think awesome, something they already have laying around can make their lives way better.
When I see things like "Nano-polmer super toilet for those without running water" I just kind of shrug. Seems to cost more than adding running water, while still not providing a clean place to drink.
What am I missing?
In 3rd world locations, you can be walking 3km every 2nd day to fill a jerrycan of water.
If that's the case then you aren't going to use that to flush.
And even if you do (using grey water after cleaning etc.), then you won't have the toilet inside the house.
How much to run a line of fresh water to the house?
If <cost of toilet> is not less than <cost of fresh water line / 10>, it seems like not a win - as fresh water can solve both drinking and help with sanitation (though not get you all the way there).
Several iterations of this device might result in something very useful in areas far away from the ocean (which makes desalination difference), and lacking the major infrastructure for centralized water recycling.