I think I proposed a similar idea in the Knucklehead Buck Driver Discussion Thread somewhere. I know I never prototyped it, but as far as I could tell it sounds totally viable. My suggestion was slightly different since it was intended to work with a LD2981/MIC5235/LT1761 style voltage regulator. I think I had a pulldown resistor keeping the SHDN pin low. I figured that a zener+resistor could be hooked up behind the normally-open momentary pushbutton and power the MCU whenever it was depressed. The MCU’s first instruction would be to latch an output to pull up the SHDN pin on the voltage regulator.