As mentioned earlier, Dan Lusthaus's stated purpose in his article "Aporetics Ethics in the Zhuangzi" (Hiding the World in the World) is to demonstrate that Zhuangzi is a "critical thinker" who uses skepticism to arrive at "positive, affirmable claims" so as to indulge in "prescriptive discourse". I also admitted that I found his arguments hard to follow, at least in actually demonstrating this stated purpose. Nevertheless, he provides many helpful insights into Zhuangzi's philosophy and the Zhuangzi generally.
On the issue of whether Zhuangzi was a radical skeptic or not, the arguments tend to go beyond me. I take refuge, however, in believing that these discussions are largely beside the point; Zhuangzi's real purpose was never to establish a rational or antirational foundation for his leap into Mystery. That would be to burn his launch pad; it would be to depend on something, and not just anything, but the rationalizing mind which he sees as the principle source of our alienation from the process of life. The "antirationalist" epitaph that has been assigned to Zhuangzi is largely correct, but to make of this a foundation for his philosophy is to render his antirationalism rationalistic. Skepticism that rests its case on a rational demonstration of its position is rationalistic whatever its conclusions. This parallels the existentialist's antirationalism that ends in despair for lack of the rational, which to say, consequent to his rationalism.
Thus, I think I agree with those who say that Zhuangzi's skepticism was primarily therapeutic, that he argued in this vein not to prove that we cannot know anything for sure, but to help us move beyond the belief that something needs to be proven. If we become absorbed in this philosophical discussion, however, we overturn the whole point of his argument, namely that life is beyond argument.
You can check out Scott's writings on Zhuangzi here.