Here's what a quick search turned up for McKenna:
"....There is something going on with these compounds that is not part of the normal presentational spectrum of hallucinogenic drug experience. When one begins to experiment with one's voice, unanticipated phenomena become possible. One experiences glossolalia, although unlike classical glossolalia, which has been studied. Students of classical glossolalia have measured pools of saliva eighteen inches across on the floors of South American churches where people have been kneeling. After classical glossolalia has occurred, the glossolaliasts often turn to ask the people nearby, "Did I do it? Did I speak in tongues?" This hallucinogen-induced phenomenon isn't like that; it's simply a brain state that allows the expression of the assembly language that lies behind language, or a primal language of the sort that Robert Graves discussed in The White Goddess, or a Kabbalistic language of the sort that is described in the Zohar, a primal "ur sprach" that comes out of oneself. One discovers one can make the extradimensional objects -- the feeling-toned, meaning-toned, three-dimensional rotating complexes of transforming light and color. To know this is to feel like a child. One is playing with colored balls; one has become the Aeon...."
On the develpement of language:
"The third and final factor, then, which pushed these mushroom-using primates into a position of ascendancy, is [that] psilocybin at the psychedelic dose level actually stimulates the areas of the brain that are concerned with the production of language, so you get spontaneous glossolalia, spontaneous bursts of modulated syntactically structured sound, and I believe probably that language was invented long before meaning as a kind of abstract exercise around the camp fire, that these homonoids and protohomonoids were doing for each other's amusement."
Hmm. McKenna's discussion is interesting. I wish he would expound on how psychedelics stimulate the areas of the brain that are concerned with the production of language or provide some kind of theory though. Maybe he does in other writings? Also, "bursts of modulated syntactically structured sound" is a fair enough description of most neologisms, but what of what I've been calling native language xenoglossy (the spontaneous expression of intelligible words in one's native language)? This later category seems to require the incorporation of meaning and grammar, and in my case understanding too, as it seems to have been evoked in response to a question and not just spontaneously. Beyond this, he seems to indicate that psychedelic glossolalia is an artifact of "experiment with one's voice", as though by humming bars you might start spontaneously forming nonsense words. This is fascinating if I'm reading him correctly; however, most of the anecdotes coming out of this thread indicate that what is said is a direct extension of what is being felt or experienced in the psychedelic state, rather than an unexpected modulation of a song or "voice experiment".
Undoubtedly you're right about cultures that have historically used entheogens having insight into this phenomenon. Maybe I'll see what I can find. I just worry they will all have different interpretations, and that those interpretations will be heavily influenced by religious dogma.