Namaskāra, Sh. Shreevatsa,
Thanks for your kind email. It may or may not be well-known, but it is certainly relevant.
I am aware of the discussion on ṣāḍguṇya in the third book of the Pañcatantra. The seventh expedient, beyond ṣāḍguṇya, is chala as per one of the two variant readings of the text.
Reading 1: evaṃ gate’pi ṣāḍguṇyād aparaś-chalo’pyupāyo’sti
Reading 2: evaṃ gate’pi ṣāḍguṇyād aparaḥ sthūlo’bhiprāyo’sti
Interestingly, later in the third book, Sthirajīvin calls his method the fifth expedient beyond the caturupāya (sāman, dāna, daṇḍa, and bheda). It seems the author of the Pañcatantra wanted to emphasise that the strategy of total deceit is beyond the textbook expedients which fall under the caturupāya and the ṣāḍguṇya. Although it may be argued that the strategy of chala that Sthirajīvin prevailed upon Meghavarṇa to follow is a kind of dvadhībhāva.
While the term ṣāḍguṇya appears only in the third book (Kākolukīya), the six expedients are mentioned by name elsewhere also in the Pañcatantra. Damanaka mentions them to Karaṭaka in the first book (Mitrabheda), and the kaulikastrī (weaver’s wife) mentions them to the kaulika (weaver) in the fifth book (Aparīkṣitakāraka). This shows that the author of Pañcatantra was well familiar with the concept of ṣāḍguṇya.
Book 1: tadenaṃ gatvā bhaya-kāraṇaṃ vijñāya sandhi-vigraha-yāna-āsana-saṃśraya-dvaidhī-bhāvānām ekatamena saṃvidhāsye
Book 5: aparaṃ mahatī kleśa-paramparaiṣā rājya-sthitiḥ sandhi-vigraha-yānāsana-saṃśraya-dvaidhībhāvādibhiḥ kadācit puruṣasya sukhaṃ na prayacchatīti
You are correct that the Pañcatantra is a Nītiśāstra/Nayaśāstra.
The other famed fabulous (pun intended) Nītiśāstra/Nayaśāstra, the Hitopadeśa, also has an implied reference to the ṣāḍguṇya in verse 2.75 (or 2.77): budhais-tyakte rājye na hi bhavati nītir-guṇavatī. The word guṇavatī is interpreted in some commentaries as referring to ṣāḍguṇya.
The Arthaśāstra, as you may be aware, discusses ṣāḍguṇya in the specific context of dealing with other kings (foreign policy).
I am currently writing an article on ṣāḍguṇya. I will share the article with the list once it is published.