an external supply would require a LOT of wires to pass through the case some how ... you'd either have bundle them, and they'd become stiff and inflexible, or end up with an ugly spaghetti mass entering the case.
why would you want an external power supply anyway, I can't see any advantage.
alternately, if it's just a board you want to power, get a car-pc power supply. they are tiny switchers (usually 100 watts or so) that fit right into the ATX socket. then you use an external 12v supply to run them. don't expect to run much of a desktop system off only 100 watts.