Thank you for the sources and thoughts. Wiltshire is making good use of the Pali sources, unfortunately he’s not too clean with the Brahmin texts and sort of throws together terms and concepts. Indeed, at some pre-Buddhist time brahmaloka was established as the highest realm, and even later (still pre-Buddhist) it became thinkable to access this realm with the suitable atman. But Vedic brahmaloka doesn’t resemble the brahmaviharas. It is accessed through proper knowledge, especially about the journey of the fabricated atman, not through ethics, love, or kindness.
As it was mentioned above, “brahmavihara” is actually a rare term in the suttas, appearing only in AN 5.192, MN 83, and DN 17. In SN 54.11-12 it is applied to anapanassati.
DN 33 calls them the four appamañña limitlessnesses, a term which appears also in Snp 3.5 albeit referring only to metta.
So it’s correct, Brahmins had the concept of reaching brahmaloka, which however rather corresponds to brahmasahabyatā, i.e. ‘companionship with brahma’ (explicitly in DN 13), and not to the so-called brahmaviharas. This is probably confusing, so in other words: Brahmins had their ideas about being reborn in brahmaloka, but it was not through love and compassion.
(Unfortunately the wiki sources of Aronson and Harvey more or less just state that the brahmaviharas predate the Buddha, but refer only to a few suttas).
What is left then for possible pre-Buddhist sources of metta, karuna etc. are non-Brahmin practices. This is particularly plausible for Magadha/Kosala where karmic retribution was prevalent anyhow. Obviously the most essential guideline of Jains was ahimsa, and it is easy to imagine that if not Nataputta then another Jain teacher would have had a meditative practice based on kindness. The suttas could then be more or less correct with historical teachers like Sunetta who in AN 6.54 is called a titthakara ford-maker (which can, but doesn’t have to, refer to a Jain).
Unfortunately, we hardly have sources of early non-Buddhist non-Brahmin texts, so confirmation will rely mostly on how much we accept the sutta references. But just to give a possible external confirmation, the earliest Jain Acaranga Sutra (I.6.5.2) contains the following:
A Saint, with right intuition, who cherishes compassion (dayā) for the world, in the east, west, south, and north, should preach, spread, and praise (the faith), knowing the sacred lore. He should proclaim it among those who exert themselves…
At least compassion and the four directions resemble the Buddhist descriptions.