To turn it on, the button has to be pressed until the bootloader is finished and the Arduino takes over to turn the transistor on ?
I like the idea, it will even work with a full board.
For power saving, a standalone is used, but with your circuit any Arduino board will do.
You say 0mA, but I don't care about milli Amperes. I want to know how much µA.
Nick Gammon has it down to 4.4µA : Gammon Forum : Electronics : Microprocessors : Power saving techniques for microprocessors