I know a lot of the Kyoto school were / have been defined as being as panentheists: but were there any pantheist Buddhists? And, has any comparative religion scholar defined it as pantheism?
Samuel Beal (1825-89), in his translation of the Fo-Sho-Hing-Tsan-King (A Life of Buddha), has defined the Vaipulya stage of Northern Buddhism as a pure form of Pantheism:
Northern Buddhism, again, may be divided into two, if not three, distinct periods of development, or epochs.
Thirdly, the 'indefinitely expanded' form, known as Vaipulya, which is founded on the idea of a universal nature, to which all living things belong, and which, by recovering itself in each case, secures for the subject complete restoration to the one nature from which all living things have wandered. This is evidently a form of pure Pantheism, and denotes the period when the distinctive belief of Buddhism merged into later Brahmanism, if indeed it did not originate it.
The following Sutras are commonly defined as Vaipalya Sutras:
The nine Dharma Paryaya texts are otherwise named vaipulya sūtra, because vaipulya literary means 'extensive'. Nine Dharma Paryāyas are generally enumerated as follows:
Laṇkāvatāra-sūtra / Saddharma-Laṇkāvatāra-sūtra
Kāraṇḍyavyūha-sūtra / Gaṇḍavyūha-sūtra
Tathāgataguhyka-sūtra / Tathāgataguṇa-jñāna-sūtra
According to the Dictionary Of Chinese Buddhist Terms:
Vaipulya is extension, spaciousness, widespread, and this is the idea expressed both in 廣 broad, widespread, as opposed to narrow, restricted, and in 等 levelled up, equal everywhere, universal. These terms suggest the broadening of the basis of Buddhism, as is found in Mahāyāna. The Vaipulya works are styled sutras, for the broad doctrine of universalism, very different from the traditional account of his discourses, is put into the mouth of the Buddha in wider, or universal aspect.
Yes, householder, the Buddha explained it to some of his monks who had been of Brahman origin. If remembering right, it is retold in Aggañña Sutta (DN27).
(Note that this gift of Dhamma is not dedicated for trade, exchange, stacks or entertainment but as a means to make merits toward release from this wheel)
I feel pantheism is a bit of a slippery term, especially when applied far outside of it's monotheistic original context. There, it started as a kind of monotheistic non-dualism, drawing on monistic and mystic traditions. But it got widely used to describe animist traditions in the 19th century, and also -I'd say unwisely- to describe Hindu thought. And in modern usage, also now to describe the idea that consciousness somehow pervades everything, and is only concentrated in minds.
Taken as immanence of the highest principle, the Mahayana doctrine of the dharmakaya is a kind of pantheism.
In the sense of coidentifying the world with deities, Japanese Buddhist thought does not put itself in contradiction with Shinto practice which is called pantheist, and as I understand it many or most Buddhists in Japan also observe Shinto practices. See also tsukumogami, for how in a polytheistic context animism and pantheism are not fully distinguishable.
Yogacara or 'mind-only' thought can I think be understood as the idea that subjective experience and mental processes like thoughts are the most fundamental way to understand reality.
But at root, pantheism as a term is not a good fit outside of a monotheistic context, and the mix of uses is prone to creating confusion. I prefer not to use the term.