I'm not SUPER-knowledgeable in pesals, but every pedal I've seen has been what they call "center-negative" when checking the polarity. I never thought about it until I got hold of my own Dunlop Power Brick, whose instructions told me to check the polarity before using it with my pedals.
Sure enough I found my oldwst pedal, a Boss Turbo Distortion, was the opposite of all my other pedals which dated from 1992 or newer. I found instructions online (possibly from Boss's website?) about how to rewire it so the polarity would line up with all my others. I don't remember the process, though... It was fairly simple, though.
I do believe the center-negative has become a sort of convention in the pedal world; there are probably those who do the opposite for some reason, though I would think that would discourage sales a bit. I certainly wouldn't want to have to purchase adapters or rewire my pedals unless it was a true vintage classic...
I'd be curious to know if I can build the "wall-wart" part of my Dunlop power supply, as the cable on mine was ripped out and sinply reattaching it didn't work... As I don't have a pedal board, I think I could make something a bit more roadworthy.