Participatory Spirituality for the 21st Century
The Indian context for "inclusivism" is actually quite broad, and covers a number of different but related contexts. It takes inter-traditional forms as well as intra-traditional. Intra-traditionally, it includes the Buddhist idea of skillful means (upaya-kaushalya) and the Vedantic notion of "differences in qualification" (adhikarana-bheda) both of which refer to the idea that specific teachings are to be assigned to specific students in accordance with their needs and abilities; the idea that certain teachings are merely propaedeutic (avatarana), which we find in both Mahayana and Advaita; the related Mahayana hermeneutic device of distinguishing literal teachings and "metaphoric" teachings (nitartha/neyartha); and the Advaita notion of "harmonization" (samanvaya), in which contradictory teachings are made consistent by reinterpreting them. All of these make use of a hermeneutic prodecure that primarily serves the theological/exegetical end of systematizing incongruent scriptural teachings found within a tradition, but it can also serve the polemical end of subordintating sister schools within a tradition. For example, in Madhyamika, the Yogachara scriptures are merely "metaphorically" true, while the Prajnaparamita scriptures are literally true; and vice versa for the Yogachara. This is actually the original context for the concept of the two truths.
Later, certain traditions start to say of other traditions that their conceptions of reality/God, etc. are "incomplete" (cf. the Jain anekantavada) or inadequate expressions of their own teachings. This idea actually has an ancient basis, as for example when the Chandogya Upanishad says that he who knows the true and absolute Being (sat), knows all teachings; or when the Buddha uses certain brahmanic concepts of the self in some of his discourses; or when the Gita says that all concepts of God are really expressions of Krishna. Later, Shankara also makes use of this idea when he says that all traditions ultimately seek the Self of advaita, but that they don't realize it (Brahma Sutra 1.3.33). Bhavaviveka may have revitalized the classical usage when he said that the Vedanta concept of brahman is an attempt at expressing the Buddhist shunyata, but due to the continuing influence of ignorance among the brahmanic sages, they don't quite get it right, and so they reify emptiness. The inter-traditional context is clearly polemical, and one might certainly question whether descriptions taken from this context can be taken or used as neutral accounts of tradition.
Wilber's system, or systems, including his most recent version of "integralism," can be understood, I would contend, as kinds of inclusivism. Among Wilber's influences, in this regard, we might include Hegel, and his concept of Aufhebung ("transcend and include"), and Aurobindo's own "integralism," his "synthesis of yoga." There is also no question that Wilber's models rely heavily on Da's own schemas, such the "seven stages," to which Da attempts to reduce the entire Indian tradition. In a note at the end of Eye of Spirit, Wilber refers to "the gross path or the yogis," "subtle path of the sants," "causal path of the sages," and "non-dual path of the siddhas," an ascending hierarchy of "paths" that clearly not only draws on Da's models but reveals both Wilber's and Da's allegiance to Tantrism. Da himself draws upon the synthesis of Tantrism accomplished by the great Kashmiri Shaiva, Abhinavagupta, in particular Abhinava's idea of the four upayas, which correspond quite neatly with Da's final four stages. Da was also influenced by the rhetorical schematizing of Neo-Vedantins like Vivekananada and Yogananda, personages whom he wished to emulate.
The inclusivism of the Neo-Vedantins is basically an extention of the inclusivism of the Advaita doxographers who follow the 15th century -- writers such as Madhava, author of the Sarva-darshana-samgraha, "Compendium of All Teachings." The Advaita doxographers presented the Indian tradition in terms of a reductive hierarchy of schools, with materialists at the bottom; followed by the heterodox Buddhists and Jains; then the the Nyaya-Vaishesika; followed by the Samkhya and Yoga; then the Mimasakas, the sister tradition of the Vedanta; followed by the dualist and qualified non-dualist schools of Vedanta; and finally, the teaching of Advaita Vedanta, the capstone of the Indian tradition (for Advaitins). Standard textbooks of Indian philosophy still use this format or something like it.
What the Neo-Vedantins do is universalize this tendency to subordinate (transcend) and subsume (include). Rather than adressing only the Indian traditions, Neo-Vedanta attempts to address all the world religions. Hence Radhakrishnan can say: "All true religion is Vedanta." Indeed, perennialism in general reveals the inclusivist tendency. It sometimes appears (or masquarades) as a kind of pluralism, but in the end it is about the dominance of some particular tradition -- whether it be Advaita Vedanta, Tantra, or whatever -- and the subordination of all other traditions to that tradition.