The opening chapter of the Laozi tells us that it is possible to speak of a metaphysical Dao, but that the moment we do so, it ceases to be a "sustainable dao". Similarly, it is possible to speak of an ultimate value (=name), but that when we do so, it too ceases to be a "sustainable value". "Sustainable" ("constant", "eternal") means reliable. Whatever we say must be in some sense definitive, but nothing we say definitively has any fixed or sure foundation; everything thought or said is "peculiarly unfixed". Kierkegaard says essentially the same thing using the metaphor of sewing without a knot at the end of the thread — with every new stitch we unravel another.
This is a practical consideration. What can we rely on? Not on anything fixed, for the simple reason that nothing is fixed. We can only rely on what is unfixed, hidden, mysterious, unknown. But relying on what is unknowable is tantamount to relying on nothing at all, and this is what Zhuangzi recommends. This is, to his thinking, the most honest and authentic response to the actual human experience. But relying on nothing is, ironically, also relying on everything. Reality unfolds and we release ourselves into its unfolding. We do not swim against the flow of events, but with them. Yet, in doing so we are also enabled to transform them, if not always materially, then at least in terms of their impact upon us. ("Events" essentially have only an interpretive reality. A tree falling in the forest may be a genuine outcome of causes with subsequent effects, but it is a meaningful "event" only when a human being sees it as such.)
This, I believe, is the core of the philosophical Daoist's vision; everything else is derivative. Let go into the great happening. Release into the unfolding. Entrust yourself to what is in any case inevitable. This also implies letting yourself happen; and this is the origin of spontaneity. And this, I would suggest, is a mystical experience in the sense that it is a paradigm shift beyond paradigms; it is a movement out of rational attempts to control life into life itself. It is also an incredible act of affirmation issuing in thankfulness. And this feels good; it is enjoyable.
All this is the "sustainable". It is sustainable because it requires that nothing be sustained. To fix oneself to a definitive Dao is to build a sand castle below the tideline; it is unreliable; it breeds the doubt that hurts, not the doubt that opens up to freedom. Thus, the sustainable Dao is no-sure-Dao.
You can check out Scott's other miscellaneous writings here.