The most notable differences in the "programming" between the two computers is in the timing curves. The automatic computers advance the timing a bit further in the lower rpms and loads. My guess is just to get the car moving a little bit easier....but i dont know for sure.
The may be other differences in idle speeds in gear and in Park or Neutral. Either way you look at it...there is no reason why you shouldnt be able to use the A9P computer...at least till you can find a suitable A9L replacement. good luck man.