The Portable's internal regulator is a switching supply designed by Apple. I don't know what the min & max input voltages are, but it will run on as little as 5.7 V from a discharged battery and as much as 9 V. The input capacitors are 16 V rated, I think, but I'd be nervous about applying 12 V anyway. Since the 6 / 7.5 V supply is stepped up to 12 V for the disk drive, it might not be able to cope with stepping down. I've been looking for a schematic, too. No luck so far. If one exists it's probably buried in some dusty cardboard filing box at Stanford.