Does any such device exist yet?
My wife and I fight over dishes - when I don't do them fast enough, she does them. Problem solved, right?
I then watch her turn on the hot water full blast and leaves it on the whole time while doing the dishes, and 85+% of the water runs straight down the drain without being used. She then gets mad when I come in and try to help, because, "I make her feel like she's killing the planet".
"No, just you're using more hot water than I use in 2 weeks of showers!!"
I think a floor foot pedal would make it easy - hands free, instant-on, waste-less.