I'm going to have to try that with my Tekkeon MP3300. I use it for both my n800 and DSLR.

Results:
The Tekkeon MP3300 has a row of 8 LEDs which indicate, what voltage is selected, and, at the press of a button, how much battery charge is remaining. My test was run using a 56 minute video (avi) of the Boeing 787 roll out. I also left wifi turned on (so omweather could update and a little use of ssh and sftp to the n800 from my PC).

After 12 hours of running the video, three of the eight LEDs on the Tekkeon were still lit up, indicating around 37% battery left, and my n800's battery was still fully charged.

It wouldn't surprise me if I got 18 to 20 hours total use out of the Tekkeon and the n800's internal battery.

(gee, I wonder how many pictures I could take with my DSLR?)