Assuming that the Reset connector is still wired similar to the older Pi, it has one pin connected to GND, and the other connected to the Broadcom chip's reset input. (Plus some passive components, to pull the pin up to 3.3V under steady-state conditions and generate a power-on reset pulse.)
See the "Dev Reset" connector shown on page 1 of the schematics, lower right:
https://www.raspberrypi.org/app/uploads/2012/10/Raspberry-Pi-R2.0-Schematics-Issue2.2_027.pdfMy suggestion was to ignore the GND pin, and have a pushbutton connecting the other pin (Reset/Run input) with a GPIO pin of your choice. That should work, I think,
if the GPIO pin really gets reset to be a 0V output when the system shuts down. And that's a big "if", I don't know how the GPIOs behave upon shutdown. If they become high-impedance inputs, for example, this simple reset idea would not work.
Edit: Come to think of it, I am not sure what problem you are trying to solve here. The only function your reset button would have would be as a "power-on" switch, right? So why not simply mount it in an unobtrusive place, where it is not likely to be operated by accident once the Pi is running, and be done with it? After all, the mains switch in the back of any desktop computer isn't secured against switching it off while the computer is running either.