EDIT: There's a question in the last paragraph.
For anyone who might be interested, here is the "before" pic of the Yamaha RBX 765A with the neck pup removed but not disconnected. The blue stuff is masking tape to guard against tool marks.
The pickup is upside down at the upper left. The gray stuff in the cavity is, I'm guessing, some kind of factory applied shielding paint (carbon?). The single black wire is the cavity ground and the light gray wire is a 2-conductor cable for the pup "hot" and "return".
And here is the "after" pic with both pups removed from the cavities and conductive adhesive copper foil tape applied.
Result - no hum and less electrical noise but there is still some noise when the pan/balance pot is either full bass or full treble.
I've seen two versions of what's in those soapbars. The Yamaha schematic shows two, series-connected coils with reversed electrical polarity in what looks like a classic humbucker. The other thing I've seen/heard (not sure where) is that it's a "phantom coil" set-up - that is, one of the coils has no magnet (or isn't near the strings) and doesn't contribute (much) to the output signal. Does anyone know for sure?