Along with improving lat strength, I think it is important that your shoulders get used to the recruitment pattern needed for a deadhang pullup. To do this, you might try uneven pullups, with one hand 4-5 inches lower than the other. This will put the lower hand into a stronger part of it's ROM, which should help assist the other arm through the bottom position. Obviously you'd want to alternate sides in a method that evens things out.
Climbers use this technique to develop one-arm pullup ability, but they use a greater distance between the hands, and specifically focus on locking out the higher hand.. I'd advise against that, as you'll fatigue very quickly.
You might need an assist from a band.. but the point of this is less about power and more about getting the feel for the movement.
I also like Grissam's idea for lat recruitment.