Remagnetizing alnico polepieces in a single-coil is easy to do. Remagnetizing alnico bar magnets in a P90 or HB, not so much. It can certainlybe done, but is not as simple as waving a 3/4" diameter neodymium over them, like you can with individual polepieces.
That said, my understanding is that magnetic "tug" is an exponential function of distance, not linear That is, the square of the distance matters.
At the same time, what you're concerned with is demagnetizing, right? If you were to "stack" those pickups so that the south of one set of polepieces was always adjacent to the north of another, it wouldn't be any different than having a bunch of loose magnetized polepieces, stacked end to end, and those wouldn't lose any strength by doing so.
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