What is on the books isn't necessarily what will be enforced, like greywater use. Personally, I wouldn't tie myself into a knot worrying about the legality of it. Don't put a 15,000 gallon cistern on your front lawn with an AK-47 painted on it.

Rainwater harvesting can also be put directly into the ground, in the form of swales, retention basins and French drains. Even grass growing below a swale stays greener without rainfall longer than similar areas without swales. I'm sure it would do nicely for a vegetable garden below a swale, too.