Got it! It was just a matter of pushing the pin up from the bottom through the center of the plastic "rivet," which is what Porsche calls the hard-to-get-at hinge cover fastener. Best of all, I managed to remove it without breaking it.
Also, only the outside hinges are covered; the inside hinges are exposed. That means fewer covers to remove, but I'll have to buy the missing covers if I want the seats "correct." I hate to fork over $150 plus shipping for two pieces of plastic, so I'll check out eBay first.