Well, I just had to jump on "mandatory" as it is one of those button-pushing words for me. I mean, even the most basic things, like respecting your fellow beings should not be mandatory. If some can't do it voluntarily, that's a problem an obligation won't fix. But since we seem to be coming from the same corner anyway, and having discussed this sufficiently, we can now throw around words like 'obligation' without creating misunderstanding among each other. :-)
I once visited a place in Northern New Mexico, also earthship inspired, with a similar setup as the one I saw in New Zealand. The guy said he didn't like semi-flush toilets for precisely that reason: that they only provide half the water for his trees. In that case he would have to water them additionally with a hose. Also, your idea of the continuous irrigation is great for using less water. Still, I would combine it with something like a worm-box, to supply the plants with nutrients too. (Recently Earthships started doing something like that in their Simple-Survival models.) So many nutrients tend to go through our kitchens, it would be ideal to hook it up with our planter.
Did you see my direct message on Slack?