Quick rundown of what I did: 

```rust
fn configure_pin_for_sense<MODE>(pin: &mut Pin<Input<MODE>>, sense: SENSE_A) {
    // SAFETY: Pin is mutably borrowed (so cannot be modified from anywhere else), and
    // pin is guaranteed to have a valid pin number (because it can only be gotten from the
    // Peripherals struct)
    trace!("activating sense for pin {}", pin.pin());
    unsafe { &(*pac::P0::ptr()).pin_cnf[pin.pin() as usize] }
        .modify(|_, w| w.sense().variant(sense));
}

```
That's the function that makes the pin sense things. It's called in the rtic init: 
```rust
        let mut button_in = gpio.p0_16.into_pullup_input().degrade();
        configure_pin_for_sense(&mut button_in, SENSE_A::LOW); // activate sense when pin goes to low
```

Then the shutdown is initiated in a task that polls the button at 50Hz (yes an interrupt would be nicer, but oh well)
This function waits for a second after bootup before "arming" the interrupt, and then starts the polling. Once the button is pressed, it waits for it to be un-pressed, and shuts down using this code: 

```rust
                info!("system off start");
                cx.shared.device_state.lock(|st| *st = DeviceState::Off);
                while button_in.is_low().unwrap() {
                    embassy_futures::yield_now().await;
                }

                Systick::delay(100.millis() /* 50Hz */).await;

                debug!("Entering System Off Mode");
                RawError::convert(unsafe { nrf_softdevice::raw::sd_power_system_off() })
                    .expect("Couldn't enter System Off Mode");
                loop { /* wait until system off */ }
```