HyperPixel which GPIO for power-on power-off

Hi.

I have a HyperPixel 4.0 screen working on a RPi2.

I would like to implement a button (or two) to turn-on and off the RPi.

According to HyperPixel Pinout, there is no free pin to do it. But I would like to ask if I could use any with minimal (or no) interference with normal functioning.

For example, could I use GPIO 3 (pin 5) and GND (pin 6) just to turn it on? After it, this GPIO 3 could be use for the HyperPixel. What would happen if it is connected to GND again while the screen is working?

In order to turn it off, which one could I use?

If this approach is completely wrong, what would you recommend instead?

Many thanks!

Grounding GPIO 3 will boot it up OK, assuming the power supply is plugged in etc.
If you edit the config.txt file and add dtoverlay=gpio-shutdown to it grounding GPIO 3 will have Raspbian do a proper shutdown. Normally that pin would be used for i2c, I have no idea what might happen with the way the Hyperpixel uses that pin? Or what might happen if you try to use another GPIO in use by the Hyperpixel? I don’t think you will damage naything but you might get some weird effects on the screen?
The dtoverlay entry can be set to use another GPIO pin for shutdown. I use the following with my Fan Shim
dtoverlay=gpio-shutdown,gpio_pin=17,active_low=1,gpio_pull=up
The Fan Shim Button grounds GPIO 3 and 17 when pressed.