Ah yes, I found this (which was previously cited from the Information for Graduate Research Fellows document):
"All Fellows, including those on reserve, must submit the Declaration To Utilize, Reserve Or Terminate A Subsequent Year Of A 3-Year Graduate Fellowship Award, each year as indicated in section 14a. When returning to active tenure after a period of reserve, Fellows may change start time from fall to summer or vice versa. When such a change in start time is made, NSF must be certain that no Fellow receives more than 36 months of NSF Graduate Fellowship support and that no Fellow is allowed more than five years from the summer or fall after the initial fellowship award announcement. Regard-less, Fellows exercising this option must enter tenure of the NSF fellowship no later than the fall of the year that their re-serve status ends."
Thus, it does appear to be possible to accept both NDSEG and NSF fellowships. However, this means that there still is no end in sight because NSF should not care if they post before or after the NDSEG acceptance deadline.