After some more searching I found Wikiquote's Zhuangzi page, which points out that the epigraph to chapter three is based upon James Legge's 1891 translation, where the passage (in Chuang Zu XXIII.7) reads as follows:
Those whom Heaven helps we call the Sons of Heaven. Those who would by learning attain to this seek for what they cannot learn. Those who would by effort attain to this, attempt what effort can never effect. Those who aim by reasoning to reach it reason where reasoning has no place. To know to stop where they cannot arrive by means of knowledge is the highest attainment. Those who cannot do this will be destroyed on the lathe of Heaven.
For comparison, here is Burton Watson's translation:
Those whom men come to lodge with may be called the people of Heaven; those whom Heaven aids may be called the sons of Heaven. Learning means learning what cannot be learned; practicing means practicing what cannot be practiced; discriminating means discriminating what cannot be discriminated. Understanding that rests in what it cannot understand is the finest.9 If you do not attain this goal, then Heaven the Equalizer will destroy you.
However, the epigraph to chapter 1 reads differently in Legge's translation (Chuang Zu II.11):
Bigoted was that Khiu ! He and you are both dreaming. I who say that you are dreaming am dreaming myself. These words seem very strange; but if after ten thousand ages we once meet with a great sage who knows how to explain them, it will be as if we met him (unexpectedly) some morning or evening.
Le Guin's quote is closer to [Burton Watson's translation)(https://terebess.hu/english/chuangtzu.html#2) but still different (e.g. "paradox" versus "Supreme Swindle"):
Confucius and you are both dreaming! And when I say you are dreaming, I am dreaming, too. Words like these will be labeled the Supreme Swindle. Yet, after ten thousand generations, a great sage may appear who will know their meaning, and it will still be as though he appeared with astonishing speed.
The epigraph to chapter 9 in The Lathe of Heaven goes as follows:
Those who dream of feasting wake to lamendation.
(I am quoting the edition published Gollancz/Orion in the SF Masterworks series. I don't know whether the typo "lamendation" was also in the original version.)
In Legge's translation (Chuang Zu II.11), something similar can be found in the following passage:
Those who dream of (the pleasures of) drinking may in the morning wail and weep; those who dream of wailing and weeping may in the morning be going out to hunt.
Burton Watson translates the same passage as:
He who dreams of drinking wine may weep when morning comes; he who dreams of weeping may in the morning go off to hunt.
The epigraph to chapter 11 in The Lathe of Heaven goes as follows:
Starlight asked Non-Entity, 'Master, do you exist? or do you not exist?' He got no answer to his question, however ...
In James Legge's translation, this passage reads
Starlight [the points of light all over the sky] asked Non-entity, saying, 'Master, do you exist? Or Don't you exist?' He got no answer to his question, however, (...)
Burton Watson's version:
Bright Dazzlement asked Non-Existence, "Sir, do you exist or do you not exist?" Unable to obtain any answer, (...)
Of course, it is possible that Le Guin consulted several other translations, but I haven't been able to identify them yet.
