I fixed up the PSU on my PC-10 II and borrowed the keyboard from my PC-10 III to test. I discovered that Q, A, S, Z, and X do not work. So I popped the keycaps off. If I poke a dowel down the hole and press gently, they work. So the problem seems to be in the switch body itself. I'm not exactly sure how these work - ie. if they are foil pads or similar. The key cap shafts don't appear to have anything conductive on them.. just straight plastic.

Is there anything I can try short of prying everything apart, which looks to be rather dicey?