Just because you buy Microsoft winbloze doesn't mean you can simply start reprogramming it. I think the idea of building your own fuel map wiht money spend on dyno and reciepts to show for it would be ideal. If you can verify that Nissan doesn't have a problem with you using there ecu as baseline would keep you safe and clear.
This is a very dangerous product many people will blow there shit up like turning the timing and turning down the fuel and then blame you for it.
BTW this same technology could make you good money as it applicable to commercial vendors and can be used on the entire OBDI Nissan family.