Both the combination and the location of the key only exist inside the person's mind. In other words, why would forcing the person to reveal the secret location of the key be any different than forcing the person to reveal the combination?
In this case we literally do have the situation where the if he refuses he would be placed in contempt indefinitely.
From the PGP software website Intro to Cryto the illustration of what it would take to crack a 128 bit key (password)
"Imagine the Earth covered with grass in a huge lawn. Also imagine that each blade of grass is a computer the can compute a billion keys per second. This cluster of computers would crack a 128-bit key on average in 1,000 years"
In this action, we must decide whether a mother, the custodian of a child pursuant to a court order, may invoke the Fifth Amendment privilege against self-incrimination to resist an order of the Juvenile Court to produce the child. We hold that she may not.
The act-of-production privilege would usually apply in such a case.