Yes, you are right, the Menu Banks in the camera, which you may have just configured, appear to be "lost" when you make a change to them, which isn't very helpful!
As you say, you can save the settings to a card(s) "reserved" for this purpose (I have "older slower" cards for this), which is what I do, and take along with me.
Unfortunately there is no way, at least that I have discovored, to "upload" the settings from this spreadsheet, for example, to the camera's card.
With my D7000 I used to set up the camera as per the settings I had configured in the D7000 Configuration spreadsheet, and then, if I was travelling, I would simply print off a .pdf of the shooting and custom settings sheets and take them with me for reference in case I had a senior moment. After a little while they lodged in my grey matter somewhere and I didn't often have to refer to the sheets (and of course there were those U1 and U2 settings).
I also stored the tool on line, so if I had an internet connection somewhere, I could get at them that way.
But the D800 seems to have a lot more things to memorize, and wont tolerate our human memory losses so well - so I may have to do some tactical re-thinking!
When words become unclear, I shall focus with photographs. When images become inadequate, I shall be content with silence. - Ansel Adams