GeoSetter Version 3.4.16 with ExifTool version 8.93
ExifToolGUI version 18.104.22.168 with ExifTool version 8.93
(With the important acknowledgement that the various metadata "fields" in my sample .NEFs will have been translated into English-readable words and phrases from the file's binary data, and that Nikon could very well use different terminology (or none!) for the "mirror up" setting from that used in the programs I list above.)
I haven't tried using ExifTool "directly", outside of those two wrapper-programs, GeoSetter and ExifToolGUI, but given the acknowledged 'power' of ExifTool, and given that both of those wrapper programs are proud of how thoroughly they work with it, I get the distinct impression that - at least from my D700 - the Mirror Up setting is not easily accessible from a .NEF file!
Maybe it's accessible in some other program(s)?!
(I set my Release Mode Dial to Mup and took some test shots, and fed the resulting .NEF files to those programs in turn; although it's a fair bet that if there is a metadata 'field' specifically for Mirror Up, then it would always be present in every .NEF file, even in photos taken with Mup = OFF. It would simply be set to some value representing "on" or "off".)
Perhaps Nikon doesn't inject it into our .NEFs at all ??? and I've just enjoyed a wild goose chase ???
Thanks a lot! I looked in the tag lists and found nothing related to MUP, nor to the other parameters on this dial which seem to have no impact on the D800 internal software (cf. S, C low, C high, shutter delay, quiet shutter and MUP). Either we use them or not, but they have no impact on image properties.