I wanted to change the colour of my Sterling pickup cover and ran into the same problem. I was told by Ernie Ball that they couldn't do it because they weren't making that model cover in white anymore. So I decided to paint it. I removed the pickup, only to find that it had been epoxied into the cover! At first I thought I'd have to mask the poles and spray, but then I realized that I could separate the glue from the cover by carefully slicing away with an Xacto blade or box cutter. It was a PITA, but it did come off. They simply run a couple of beads of glue, one down each side. I don't think there was any on the ends. Once the cover was off, I roughed it up with some sandpaper and gave it about 8 coats of paint. It turned out pretty good. Make sure to use a matte finish paint, glossy probably wouldn't look very good. Recently, I noticed on eBay, there are some cheapo noname pickups available that have white covers. I bought them just for the covers. Who knows if they'll fit, but they were billed as MusicMan replacement humbuckers, and for $15 I was willing to roll the dice...
As for corroded pole pieces, I have taken one of those sandpaper sponges and lightly rubbed away until shiny... Seemed to work pretty good. Remove the cover before you do it though.
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