The live view of the D800 is designed to provide a faithful idea of the resulting exposure.
In landscape situations, the simplest workaround to brighten temporarily live view is probably to switch from photo live view (the target correct exposure is set) to video live view where a slower shutter speed is set to make focusing easier.
So changing the shutter speed in live view doesn't affect the brightness of the live image? What is the difference in shutter speed between the correct exposure according to the meter and an appropriately "correct" (in brightness) live view image?
I really should get my hands on one of these things but if you have to change modes to adjust the exposure/brightness of the live view I can see how this would be frustrating to Canon users who are used to exposure simulation (which works perfectly with Canon lenses, less perfectly with non-Canon lenses).
Does the d800 come with the required tethering software in Japan?