Similar to this request to add shortcuts for 
+ 
, + 
, etc.
It would be a total game-changer if we could handle the Lock button 
as an arbitrary input, instead of its normal behaviour of locking the screen. Naturally, a permission would need to be granted by the user in order to do this. And the user would always be able to lock the device normally by first opening the menu and then using the lock button ( → ).
Having just two buttons is very restrictive for design. But access to a third button would open up so much. Obviously, the lock button’s position would mean it should be an auxiliary button delegated to rare actions.