Hello Squalish,

Thxs for the informative reply.

I think you have a winner with 3), btw - it could shave off perhaps 10% of daily water use in areas that import water at great expense.
Mandating that all toilet water comes from greywater could also be combined with things like a 'urinal flush' lever, and be granted an exemption on the 3.5-5 gallon toilet ban in the US, which is incredibly annoying and doesn't really save that much water - it just makes you flush your statutory 1.6 gallon toilet twice or three times.