[marktwain]

I have often considered making a rig like that with the Bushman knife sold by Cold Steel.

It has a hole in the handle which could be used with a detent to lock the knife onto a shaft.

I have not actually made such an item.

Here is a video review and demo of the Bushman.

[Rebelbase]

That looks like a feasible way to make a detachable spear.

My idea is to weld the male end of a quick connect fitting to the butt end tang and have the female mounted on the end of the shaft with some sort of cover over it to keep out dirt.