"In order to shoot AF-C focus priority, and in order to be able to do a focus/recompose, I *need* two buttons- AF-ON and an AF Lock button."
Bought D600 today and immediately ran into this problem.
PERHAPS a solution: Program the FUNC button (on front of body low down) to AF-ON. I did this, and can still use the AE-L AF-L button (which is of course programmable for many uses) and ALSO the "OK" buton to center the focus point when shooting AF-S.
Shot 50 or so quick shots at lunch and was OK with this.
OF COURSE, I'd rather have a dedicated button, but this way my fingers and thumb can easily find (I tried with eyes closed to success) these 3 controls. (Still programming shutter 1/2 press to lock exposure so I can recompose).