I've seen openings for the RC car, but not the one at nuketown. I find this one quite odd - Nuketown doesn't seem like a very system-intensive map design, so I see no reason it could not be done. Maybe it was a last-minute addition? It could also be because the jump requires you to use the boost (I have never boosted the Wii RC-XD...is it possible?) so putting that track on Wii would require re-designing it without the jump.
However, I know there are RC shortcuts in the game, but I have never used any of them - after a couple days of playing, before I knew about the shortcuts - I stopped using the RC.