I'm surprised to see water conserving toilets but not a water conserving shower. Most showers today only have an on/off valve. To get the desired amount of hot water you turn the valve till its get to the desired temperature, but there is no way to correct the amount of water.
Why not create a small button that allows the water amount to be cut by 1/3 or 2/3. This can be used when one is soaping themselves.
When rinsing you can use the full force and of the shower, but when applying soap, instread of turning the shower off and risking messing up the desired temperature settings, you can press this button and only a sprinly of water would come out.I feel that more attention to conserving water in the shower, rather than the toilet will allow for greater conservation of water.