Got the power off working:
Code:dtoverlay=gpio-shutdown,gpio_pin=19,active_low=0
I then have my momentary switch between 19 and 3.3v on a rail (provided by the 1wire connection).
It doesn't do power on, but that's ok.
WRT an LED - The 5V outputs on the Pico board are always hot, it looks, since power to the Pico is independent of the state of the Pi. This is actually good info - means that even if the Pi shuts down/reboots, stuff will stay on.
Are there any pins on the Pico that are tied to Pi power at all? I can go the USB route, but space is tight to get a connector in there
Check the 3.3V line coming back from the Pi - it’s unused on the Pico board (outside of a capacitor).