We've been flushing with rainwater these last few months (until the recent long dry spell!) with a cheap 12V pump in a water butt, float switch in the cistern and avoiding modifying anything in our rented house by shoving the wires and fill tube up the overflow.
It works a treat:
Rainwater toilet flush
All new houses should have systems like this installed as a matter of course - it is just so wasteful (& expensive) to use drinking quality water for flushing the bog.
Another thing to add to the list for the long-awaited self-build.......
My old man rigged the downstair loo up to a water butt with gravity feed.He also rigged a caravan style pump up to a micro switch as back up because the gravity feed is a bit slow..The caravan water pump is powered by a battery charged from a solar panel.I dont think it cost him many pennies to make as he has most of the bits.