Nearly all humbuckers have the magnet underneath the coils in the centre where both coils meet between the screw polepieces and the slugs. This way the screws are one polarity and the slugs are the opposite. The only pickup I know that was really different to this is the original PRS, where the slugs are magnets.
Also P-90's have the magnets underneath and the screws conduct it through the coils. In this case (and with some high output single coils), there are 2 magnets, one either side of the screws.
You could only try your idea in a single coil, like a Strat. I had a similar idea last year, but using a mix of Alnico IV and V and changing between the bridge (E,A,D,G -V, B, E - IV), middle (E,A,D -V, G, B, E - IV) and neck (E,A -V, D,G,B,E - IV). Tim has done some experiments with it I think, but I am not sure how it worked out.
