I'm not sure yet whether the overall behaviour around widgets in the outside bits of contraptions is intended or not, but I do know that one of the design considerations on Decker is that decks should still be usable on touch-only devices, and not require keyboard input. So I think the intent is to not be able to have keypresses without a corresponding clickable button.
Hiding a button off screen might work as a workaround though if you really want to do it. But if you're targeting web decker and don't mind diving into the danger zone, there's also functionality in the forbidden library to read keypresses directly from javascript.