Originally posted by Alex:[..]
Sorry, but I'd guess Mr. Page either fell for some kind of self- induced voodoo mojo placebo effect or the pickup covers from the good old days of rock and roll were made of inferior metal.
Keep in mind that guitar players and HiFi fanatics believe in almost anything that's said to improve the sound. There's a gazillion topics about overpriced sound gizmos and electronical wizardry that no one really needs.
I saw Page and Plant live on the Clarksdale tour, and though I hate to say so - the guitar sounded awful. In fact everything sounded awful, and all that was audible consisted of an overloud bass drum, screaming vocals and a mumbling layer of sound mud.
Alex
Live sound is naver really that good...anyway, it could be that he was just experimenting to see if it made a difference...or just to change the looks...I don't know. I am actually just commenting...I am no guitar player. Maybe we can find a blindfol test on this issue...that would be interesting.
But as you say, theorically that in-serie-connected double pickups shall sound like a humbucker. Nevertheless I would be seriously worried about the possible humming and feedback, you'd better be careful while setting up the pickups or that would sound like a plane taking off 

I just pointed the risks of out-phase and humming problems, but I see you're aware