>I just ran into this same thing, and found in Darrell Young's >book that the battery must have at least a 50% charge or you >cannot access this function. I recharged the battery, and now >it's fine. > >Dave
The battery does need to be charged above 50% (higher is better) in order to access the Mirror Lock Up for cleaning menu item. This is not the issue I'm referring to.
This is a firmware bug in the D800/E and seems to occur after altering camera settings in the 8 available banks. Even with a fully charged battery both the Mirror Lock Up for cleaning and the Dust Off ref photo menu items become grayed-out. The solution given to me from Nikon was to temporarily change to the Custom Settings bank 'D' to access these menu items, and it works.
Again, not all users of the D800/E will experience the problem immediately, but will encounter it eventually.