Seymour Duncan makes a true single rail pickup. I saw it on musiciansfriend.com just yesterday! Look under "pickups", narrow it down to "Seymour Duncan" and keep looking... you will find it. Though you might have to do some funky polarity reversing or something to make it go with the DiMarzio 'buckers.

I'm thinking about putting it in the neck position of my H-S-S Strat. For the middle pickup I want something surfy/jazzy/countryish... and then for the 'bucker I'm stuck between the SD Distortion and the SD SB. My only worry is whether either of them will worked on the F-spaced Floyd Rose tremolo setup.