I know the 7 inch screen swap has been done on a lot of Nissans but you run the risk of losing your Infiniti Information Screen. As for retrofitting the Infiniti Navigation, it could be possible even some people believe its not possible. But I would imagine the parts alone would cost easily $3000+ and wouldn't be worth the trouble. If you had put an aftermarket screen, you would loose the Infiniti Functions and Info display and/or would have to keep it tucked away behind the radio somehow. That's usually how it was done with the other Nissans, I know many people did it for the 6th generation Maxima's for sure, but then again it was a lot less complicated too and it seems the newer infiniti's you have more too loose as far as factory functionality because there is so much in it. If the loosing the factory interface is okay with you, then I don't see how a little creativity won't stop you.
Update: Maybe this will help you a little bit?
http://www.youtube.com/watch?v=M1O6PIEs ... re=related