smart, I just played around with it, I can use the shortcuts app to create an action (like turn off the lamp or 'identify music via shazam') and then go to accessibility->touch settings and use double tap to trigger the action.

It works without unlocking but the phone has to be awake, so I can hit the power button once to wake and then double tap the back to trigger a shortcut without unlocking, confirmed to work without needing to identify me via face ID or anything.

There is also an accessibility shortcut for triple-clicking the side button but it only allows for toggling accessibility features.