Couple things both in the same menu area: - You could try ==> Custom Settings Menu ==> f) Controls ==> f9) Customize Command Dials ==> Aperture setting ==> Aperture ring This would allow Aperture ring to control Aperture setting.
Independent of the above you could also try ==> Custom Settings Menu ==> f) Controls ==> f9) Customize Command Dials ==> Change main/sub ==> ON. Then see if the back command dial will now change Aperture. Perhaps the problem is worked around by switching to the back dial for Aperture?
If you don't mind, I and perhaps others would be very interested in your impression of the D800 video files.
BTW - My Nikon G lens (no aperture ring), works fine, I can control shutter and aperture with front and back command dials.