See if this makes sense...
If you have a small, drinking water system, say 4g. At best case you would produce 4g of 'waste' water for every gallon of drinking water. The only way the waste water from the RO system will flow into the hot water system is for there to be room in the hot water system. This happens every time we turn on the hot water to shower, wash our hands, clothes, dishes, etc. We probably use much more hot water than we do drinking water, so in the zero waste system the RO unit will only produce water (filtered and waste) when hot water is being used. This should work fine if you are storing and using a small amount of drinking water. In the 4:1 scenario the zero waste setup will only be able to produce 1/4 the amount of filtered water as the hot water you use every day.
In my house I often use 30+ gallons of RODI water in a day, not every day but often enough that I need it available. That means I would have to use120g of hot water for every 30g of RODI water I use. I know that my household can't sustain that kind of hot water usage. So the zero waste system won't work for me. I like my RODI to be as on demand as possible, therefore it runs 24/7 and maintains my auto top off and keeps a 30g reserve on hand. An auto shut off is the best way I know to minimize waste.
In the case of the inline RO membrane, I don't see how you could cut your waste in half. I would expect and increase in production but I don't see how it could be as high as 50%.
Warming your input water will make your RODI system more efficient. Melev suggests coiling input tubing through a 5g bucket with an aquarium heater to achieve this. I don't know the math on this but I would expect to use enough electricity to offset any savings from the efficiency gained.
RO works best when it has good pressure (usually 60psi and up). Booster pumps are needed if your city water does not have enough pressure but standard for most places is 60psi.
Don't overlook basic maintenence for efficiency's sake. Keeping good filters and membranes will make any RODI system work better.