That's a very interesting problem!
I think it could be done with transmitting and receiving transducers on a belt. An ultrasonic pulse, generated by an MCU, which resets a timer, is transmitted. An MCU with receiving and transmitting transducers, on, say, the right wrist, detects the pulse, and immediately retransmits it. The belt MCU detects the retransmitted pulse, and stops the timer. From the speed of sound, and the time for the outward and return journey, the total distance can be calculated. Halving the distance gives the distance from the wrist to the waist.
The distances are relatively small, so enough signal should be present, even if the transmitter and receiver are not line-of-sight. Amplification will be needed for reception.
I'll leave you to work out the details for all four wrists and ankles. :) Some form of encoding will be needed so that the four remote units can be distinguished from each other.
A Google search for similar techniques turned up this, which uses a passive reflector on the hand, with the transmitter and receiver on a microphone stand. I can't see that simpler technique working for Glen's application.
Depending on the type of your door and key, you might have some success with inductive measurement from the inside. If your door, lock, and cylinder are mostly diamagnetic, and the key is ferromagnetic , you can measure a change in inductance of an inductor mounted on the inside of the lock when a ferromagnetic key is inserted. This should work for paramagnetic keys as well.
If your key is ferromagnetic, you might successfully magnetise the key and measure the inducted electromagnetic field when the key is inserted.
If the door cylinder is useable from both sides, you might insert a pin into the lock from the inside that will be moved when a key is inserted on the outside. Not a good idea because it might possibly damage your lock, though.
A totally different way would be to force people to put their keys on a specific hook, and alert if the key is not there.
Or make the door keyless.
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How about a tilt switch? It gives a true-or-false output only, which seems to do the trick unless of course the "card" you mean is very thin.