Re: would this work in a pickup?
The metallic compounds that make up Alnico magnets are very stable, so the theory that they degrade or age is largely false. However the magnets can become demagnetized when exposed to strong magnets or sources of high voltage. So the question of why a vintage pickup sounds different than a new one, probably has something to do with the magnets being degaussed when the guitar is leaned against an amp or cabinet.
The potting material also breaks down over time because it is mostly paraffin wax. When the potting breaks down the pickup becomes microphonic. A vintage pickup may also not have been potted to begin with. It is also possible that oxidization of the copper winding may change resistance or capacitance slightly.
