So it looks like there's no way to save that 9th Nikon PC (D2XMODE3) in the camera.
I tried several things, including changing the file extension from .nop to .ncp, thinking it would install in one of the 9 custom locations. But the D700 still recognized the file as one of Nikon's PC's (vs a custom PC) and only offered the option of installing in location 5-8 of the first 8 (which already have Portrait, Landscape, D2XMODE1 and D2XMODE2).
I give up.
Does anybody know if there is an inherent difference between PC's with the .nop vs the .ncp extension, other than the extension telling the camera where to install it? (i.e. in one of the last 4 open locations of the first 8, vs C1-C9)? There must be, because changing the file extension didn't work.
Anyway, I was able to install all of them in Capture NX2, and will just have to apply in PP, not in-camera.