Would have to ask those who do flush every time, whether they are metered or not. I'm metered, but that's not the only reason I would flush every few times (unless expecting a visitor), it is a major waste of good clean drinking water.
If we had some tank arrangement built into the upper half of the outer walls nearest gutters (ie a foot or more above lever of a low tank loo (presumably the majority of toilets these days), then we could use rainwater to flush (subject to a bit of filtering to reduce change of clogging from moss, leaves, etc). A neighbour would love to propose this for 'green living' to the major house building groups, and I feel it could be adapted (with tanks on each floor, in only outer walls) to cope with 3 floor homes with multiple loos.
Obviously it would need some form of insulation to prevent freezing, and careful design so access could be achieved from outside for replacement/ maintenance in case of any leaks... I suppose it would become inexpensive to add tanks along with cavity wall insulation (or whatever is currently expected in new build homes, I'm not up on all the regulations!). Also, house structure and load bearing ideas would need revision.
Indoors if one assumed the tank was say 15-24" "thick", the whole depth of room (from window towards centre of house, say), and perhaps 4' tall (from ceiling down), then below the tank there could be storage cupboards, shoe draws, etc, and then pipework from floor to floor [for ground floor tank to loo and/or garden tap], and to the loo on the current floor, would be extra to plan. Some form of valve could be used at each loo, so in times of no rain, mains water pressure would enable it to be used without human intervention to switch anything on/off.
If having a gap under these tanks was a problem (I do appreciate there's some significant weight involved, and in most lofts there is good support to handle the main water tank), then having full height tanks, perhaps a bit narrower, may be possible, still with some mechanism to ensure top-most tank keeps some water and 'overflow' goes down to next floor, rather than having such high pressure at the ground floor that there'd be massive problem if garden tap washer was leaky, and all floors lost all their water via that tap...
Any builders / architects care to comment?