Now he can "simply" adjust the whirly-weighty-thingy to suit; higher for longer string, lower for shorter string. Ha!
That's a good idea poptop, it hadn't occurred to me yet.
. . . has anyone every fabricated a top wherein a weight such as that was free to traverse up the axle?
Do you mean a weight that would move up or down while the top is spinning? No, I don't know of any top like that. The Jupiter adjustable center of mass idea was inspired by pizquie's prototype and he just did it to experiment.
Also, has anyone included centrifugal mechanism (e.g. like a centrifugal clutch) to allow the weight to be distributed outward?
There have been tops with mechanisms like that to protrude spikes for fighting or wings for flying, but not to adjust the playability. Moving a weight out will slow the top and moving it in will speed it up. But the total rotational momentum doesn't change nor would the spin duration (neglecting friction changes).