I did the Assault on Mt. Mitchell back in 1997. I'm trying to decide if I'm stupid enough to go back again...

I don't remember where that cut off point is, because I don't recall it being an issue in 1997.
Looking at the web site, I think the cut off is at 86.7 miles, approximately 12 miles after the serious climbing begins, and 9 hours after the start of the ride. If that cut off was in place in 1997, I would have passed that point by 3:30.