I have experience with many upgrades, installations and repairs, but this was idea was just outside of my usual area of expertise. It’s an interesting mod that appeals to those who want to save the environment without sacrificing their wallet.
The DIY fix here is to make your existing toilet a Dual Flush mode toilet. The idea is that it uses less water for flushing liquid waste vs a normal flush for solid. The mod kit in the link below costs $20 and can save up to $100 a year per toilet. Check it out and tell me how it goes.