It already is, though not for hotel room doors/etc. I used to have access to a colo where racks rented individually (i.e. not in a cage) had a combination lock

It already is, though not for hotel room doors/etc. I used to have access to a colo where racks rented individually (i.e. not in a cage) had a combination lock that was electronic. To use it, you first "charged" it by turning the knob back and forth a few times, then entering your code. The correct code mechanically connects the knob to the locking mechanism, so turning the knob unlocked the door. In the case you mentioned, I do wonder if you could get a bit more energy from the door itself rather than just the handle. For example, if the spring-loaded self-closing mechanisms weren't just mechanical, but used that energy to generate electricity, you might get a bit more out of it.