I wonder if there's a way to beam the key out into space with RF at a very distant, but RF-reflective, object. The key hits the object and bounces back and cannot be received until it reaches earth again. I'm not sure such a set-up is physically possible but I think it would be a truly timelocked situation.
Except if someone can sniff your outgoing RF... given that the power reflected back from the very distant object would have a gigantic attenuation (just the path to the moon has ~250 dB attenuation), you would have to use an enormous RF output power. But at this point, anyone close enough to the transmitter (even if you use highly directional antenna) could basically sniff the key being transmitted.
