In his essay "Bimodal Mystical Experience in the 'Qiwulun' [chapter two of the Zhuangzi] in Hiding the World in the World, Harold Roth tells us that Zhuangzi's philosophy is part of a larger community of mystical practitioners of 'breathing meditation'. I'm not so sure.
Roth offers several passages which would seem to support the view that Zhuangzi was aware of and probably practiced this form of meditation but largely simply refers us to his other studies on the subject. My issue with Roth in this regard is not primarily about whether Zhuangzi did or did not practice this form of meditation but that in so completely immersing him in this tradition he also imports all manner of religious baggage utterly alien to Zhuangzi's philosophy.
We are told, for instance, that Zhuangzi teaches that one can "merge with" a "force" that he calls "Dao". I read no such claims to union with a metaphysical Dao in Zhuangzi.
The "Illumination of the Obvious", Zhuangzi's simple appeal that we take a phenomenological approach to our assessment of reality, accepting, for example, that our rights and wrongs are a function of our perspective and not cosmic realities, becomes for Roth an "enlightenment" in the usual sense of the term. We are awakened from the dreaming, not to it. Ziporyn explicitly suggests just the contrary, that it has nothing to do with an illumination of the mind: “Some take the term . . . to indicate a higher, perhaps intuitive type of understanding, which transcends the relativism of perspectival rights and wrongs, a ‘Great Knowledge’ somewhat similar to Buddhist enlightenment. But this seems inconsistent with Zhuangzi’s relativist critiques . . . It then refers not to a deeper apprehension of the real, transcendental truth lying beneath the surface of appearances, but rather to attentiveness to the surface itself, the most obvious and undeniable feature of which is the disagreement between varying perspectives and their transformations.” (Zhuangzi; p. 218)
"Being empty" similarly becomes for Roth an emptying of the mind of all thought. Again, this is, I believe, a complete misreading of Zhuangzi facilitated by squeezing him into the box of traditional mystical praxis.
It is likely that Zhuangzi practiced some form of meditation, but what I find much more telling than conjecture about what that was is that he does not find it necessary to tell us. Open any book on "mindfulness" or any other modern form of spiritual practice that advocates meditation, and you are likely to read about little else. However important meditation may or may not have been for Zhuangzi, it had clearly not become an obsession. And this, I would suggest, is because he did not see it as a means to attaining some enlightened state in which one is "united with Dao". When Zhuangzi says "It is just being empty", that includes being empty of the belief that one can somehow be saved from the fundamental existential realities of the human experience of drift and doubt. Our experience of Mystery is always of a “Shadowy Splendor”.
You can check out Scott's writings on Zhuangzi here.