Ok, I just tried to take my Manfrotto 680 apart.
No-freak'n-way I can get the top off. I have motorcycle tools, and I have a fork-tube vise that can clamp pretty well. The top of the 680 won't budge even with a good amount of force applied!!!
My insides look like this...
I think I am going to sequester-out of this, because at this point, $25 is not worth the effort.
It appears, that Manfrotto presses into the upper monopod the head - but I'm not 100% since most manufacturers screw the top on. It could be grey lock-tight up there, but without a exploded parts diagram, I can't honestly say how to take this apart.
Worst case scenario, If all else fails, I might be tempted to cut below the top where the stud is, get if off, and see if I could make a simple replacement.
Dunno. I'm out here since I don't want to recommend a fix that destroys everything.