I don't have the D800 nor the 5D, but have envy. ;-)
Video's aspect ratio is 16:9, so reapportioning real estate would have to happen anyway to use all the information in each pixel, you wanted to resample the pixels, which isn't something I would want to do. Resizing the amount slightly from the chunk available makes a bit more sense.
While 35x19 sounds ideal, my guess is that engineering plays a huge role in the art of the doable, assuming that Rockwell's information is accurate.
Finally while the 5D might use the breath of the whole sensor, it, too, is cropped to meet aspect requirements, and but only a third of the lines available are sampled for video. I don't know how Nikon does their sampling. We might find out they discard lines in the sampling process, too.
Keep in mind, high definition video is just 1080x1920, the camera isn't going to put out more than that so it would have to be sampled down, there is a lot to lose and still look terrific.