The problem I see with this is that, if the 12 stops of the 5DIII isn't enough, then the 14 stops of the D800 is unlikely to be sufficient, either...and, in any case, you'll get far superior results with even a two-exposure HDR rendering.
Now, consider that the 5DIII has an in-camera auto-HDR mode that will do a 6 frames per second bracket and even give you a not-miserable JPEG preview of the results...and the dynamic range advantage of the D800 vanishes in a flash.
That leaves the only remaining real-world advantage to the D800's dynamic range being for high-contrast scenes not suited to tripod work where you'll still have the luxury of being able to do significant amounts of post-processing...and, for the life of me, I can't think of anybody who actually shoots like that. Outdoor daytime sports published in high-end magazines? If so, good luck getting the shot in the first place, between the D800's framerate, autofocus, and Nikon's heavyweight supertelephotos....
Do you shoot?
Do you print?
Do you do ANY post-processing to make your print or screen image look BETTER?!?
If so, then you'd appreciate the value of the extra DR. After all, especially when printing, you have to compress the DR of the captured scene to very limited effective DR of print media otherwise you have a very contrasty image with blocked up shadows and blown hilites.
Of course, if all you do is get your snapshots printed at the local drugstore then you'll likely never realize the benefit of a superior DR image if you don't know the value of it in post-processing.
And how is a high-contrast scene not suited to tripod work? Do tripods melt in the summer sun?
I still like my Canon gear, especially the modern glass, but I'm ecstatic about the results I get with my modern Nikon bodies, even using 30 year old lenses on them, because the noise levels are so much cleaner at low ISO. The more I use and compare the two camera systems, the more I see the weaknesses in Canon's noisy low ISO images, even when controlling the lighting to some extent.
Sure wish I COULD mount my Canon glass on a Nikon body tho. And I DO actually prefer Canon's ergonomics and user interface but the Nikons get me the file that make the prints that makes me smile.