Cutting away the bare minimum from each of the "plastic pop rivets" on the bottom of my Powerbook keyboard revealed the membranes nicely. It turns out the thick traces are indeed for the caps lock LED.
This is as far as I'm planning to take it down. It turns out 1995 Apple made working on this keyboard as hard as modern Apple would. In addition to the destructive removal of the aluminum plate, the membranes are bonded together in two places, so I'd have to cut that part to separate them fully. I hope that won't be necessary.
Some of the pads definitely suffered corrosion from the battery leaking in the case. Very careful application of alcohol cleaned them up ok, but the missing keys still didn't register. After some poking around the affected traces with a meter, it turns out I'm missing some of the matrix because the traces have become magically non-conductive in a couple places.
So how does one go about repairing that? Fortunately, while this is new territory for me, it's hardly a new issue. A quick search returned an eevblog conversation that recommends conductive ink. The good stuff from MG Chemicals isn't cheap, but I'd rather pay a bit more and have a better chance of success. Time to work on something else while Amazon does its thing.