Well, I have to admit I put the Fedex logo and colors on a model (Multiplex Sonic Liner) nearly 5 years ago and have been flying it regularly at events since then. I never asked permission.

No one cares, except for the occasional Fedex employee who thinks it is pretty cool.

I would decorate the model as you wish and let the chips fall where they may. At worst, they MIGHT tell you to not do it when they find out about it and I personally think your odds of winning the Powerball lottery are better than the odds of Nissan ever saying anything about it.