Hillary, Sorry - with my first post, I did not see that you had already clarified your original question. Obviously, if you are considering buying the D800, and don't already own it, my suggestion to try something out does not help much! But, the article from Thom Hogan may answer some of your questions if you really do need to do continuous recording, especially if it is over 2 hours.
Since the LCD monitor and HDMI output are not limited by heat, it is the sensors being powered up that might create a heat problem that could create a cap on what you want to do. I can't speak definitively on that issue.