After my lesson on Thursday night I had decided to stick a humbucker I'd gotten off eBay into my strat - I'd never much liked the sound of the pickup in the bridge position, so I found an H-S-S pickguard and an inexpensive humbucking pickup. Here you can see it just screwed into the pickguard (alongside a little block of maple I may someday use to fully block the tremolo):

So Saturday afternoon I finally grabbed the soldering iron and wired her up! Things went relatively smoothly - I don't like working with four-conductor cable, though. The sub wires are very tiny (especially the bare ground) and seem far too delicate. But so far everything has worked, except initially I wired the humbucker out of phase with itself because of a misunderstanding of the Seymour Duncan wiring diagram, which caused it to sound like a hilarious choked ping instead of a beefy humbucker.
All I had to do was swap red and black and everything was fine. I even wired it so that the middle + rear position on the pickup selector selects only one of the humbucker coils, so it keeps that classic strat sound. Here is a picture of me scrutinizing wiring diagrams:

Here is the finished product sans the control knobs. I think it looks snazzy, and the sound is what I was hoping for. The output levels aren't uncontrollably different, and the bridge sound is much better than the sproingy single coil pickup - it has a thick controlled sound that is much more SG-like.

Coming soon: new bridge saddles from GFS + a Floyd Rose style bar string retainer.