Thanks, guys! With gritty resolve, I went through a few coins again. Using my Leatherman Wave to get a firm grip, and biting down on my belt to avoid cracking any teeth, a 1996 quarter finally did it. And I think I prefer the fit in the neck sheath without the clip. A lot easier to remove, and the knife feels better in the hand.
I wonder why they crowned the slots in the screw, rather than making them flat-bottomed? Seems like the latter would be more efficient, and it's easier to find hardened screwdrivers than it is to find hardened coins.