A dummy coil is exactly like a regular pickup except that it is wound in the reverse direction and has no magnets. In an American Strat, the middle pickup already has reverse winding, but I don't know if there's any way to remove the magnets. I doubt you could get a magnet-less pickup from Fender, but smaller shops (maybe even Duncan if you write to them directly) can probably produce something like this for you.
Your middle pickup now is probably reverse-wind, reverse polarity. When you run the guitar in the 2nd or 4th switch position where two pickups are on, then you are getting a hum-cancelling effect, but you're also getting a different tone because there are two pickups on. If you had a dummy coil, the neck pickup would sound exactly as it does now but without hum which is good, but you'd lose the in-between tones.
In the 80's Fender made a Strat Elite model which had 3 special single-coil pickups with a 4th dummy coil in the back of the guitar. I had one of these which was a good idea but not a particularly great sounding instrument. The original Paul Reed Smith bass also had a setup like this with 3 single coil bass pickups (theoretically like a Jazz Bass + EB0 pickup) and a dummy coil in the back as well.
David Fung
