Here is yet another usage to show that the wod 'nimitta' can mean upAdAna kAraNam:
There is a work 'AtmapuraNam' authored by Sri Shankarananda Saraswati believed to be the Guru of Swami Vidyaranya. The work is a compendium of several (10 plus) upanishads in verse form. It has a very useful commentary too. In the Aitareya Upanishad there is the statement: The Three states, waking, etc.are three dreams. While explaining the dream state the text says:
तत्रात्मनि तथा दृश्ये नियतं नैव पश्यति । तत्र दृश्यो गजः क्वापि तरुर्भवति तत्क्षणात् ॥ २०४ ॥
तरुश्च पर्वतः क्वापि पर्वतश्च क्वचित् तृणम्। एवं द्रष्टा क्वचिद् विप्रः शूद्रो भवति तत्क्षणात् ॥ २०५ ॥
क्वचित् पशुः देवो महाराजः क्षणात् | देशकालादिनियमो नात्र कश्चिद्धिं विद्यते ॥ २०६ ॥
In the dream one will not see regularity (niyatam) in respect of himself and the observed world. In a trice an elephant can become a tree, a tree a mountain, that a blade of grass. The observer, a brahmana, could become a shudra, an animal, a god, a king in a trice. Thus there is no regularity seen there with respect to place, time and causality.
While explaining about the causality aspect, the commentary says: मृद एव घट इत्यादिनिमित्तनियमः |
(there is no rule that a pot is produced from clay alone).
Here the word nimitta is used to convey the idea of material, upAdAna, cause.
One can see an exactly similar usage made by Shankara in the Adhyasa bhAshya: mithyAjnAna-nimittah.
Since the word 'itaretara avivekah' has already been used there itself to convey that the aviveka, non-discrimination, is the nimitta, instrumental cause, the word nimitta in the next compound word mithyAjnAna-nimittah has to be decidedly mean upAdAna kAraNa for the bhrama/adhyAsa.
What was explained in the earlier post on this topic is now reinforced by a very unambiguous analogy shown above: nimitta = upAdAna.
Those who have not read the earlier post may please do so for greater clarity of this post.
Incidentally, a study of the Atmapuranam along with the commentary could turn out to be extremely rewarding, especially for those who have not studied the Upanishad Bhashyam.
Om Tat Sat