[Shane826]

Bill said “ In other words, you need to be able to push a bit of cold water through the cold water line to the toilet, without filling the water heater. And you need to do this only if you change the toilet. To do this, and avoid the whole bypass thing, would it not be possible to put a simple shutoff valve in the existing cold water feed to the water heater, thus avoiding the whole "how many gallons of antifreeze to fill it"?”

Valid point. But if you’re going that far, why not install the second valve with a hose in between the two and push the pink through the hot side as well?