Try Custom Setting c4. The D800 allows you to set up the monitor off delay for live to "no limit." As I understand it, this should work for playback of video or a slide show of stills, but I have not personally verified it goes longer than 2 hours. Give it a shot and see if it works.
To be sure I understand your question, you are talking about continuous video playback and not recording for 2 plus hours - correct? If you are talking about continuous recording from the D800 throught the HDMI port, check out Thom Hogan's article on this - http://bythom.com/hdmivideo.htm . I've never tried it, so I can't offer any first hand experience.