I seem to have lost a post to the Gremlins Otto's World that was a follow up to the last post. It may shed some light on this discussion and address some of your points, so I will attempt to re-create:
I started by saying: I think I disagree with Tolkien that Art (Fantasy) is to be distinguished from Dreaming or Madness/ Hallucination. I imagine that if he and McKenna could converse Tolkien might be persuaded otherwise. Fantasy is created from the imagination with conscious (but also
subconscious) intent. Dreaming and Hallucination are expressions of the subconscious imagination, considered spontaneous and uncontrollable. Yet Dreaming and Hallucination may be entered into with
full awareness (consciousness), as through Lucid Dreaming, and as through paying attention to the distinction between the hallucinatory image and the "Real" World upon which it is superimposed. So Fantasy and Dreaming/Hallucination are both expressions of the "imaginal realm" in their own right.
So I think Tolkien might come into agreement on this if only the opportunity existed to converse with him.
And to add to this in light of your last post: Visions of the Prophets, often considered Madmen can be seen then as messages from this other "hidden" realm. And the Muse, related to Sophia (the personification of Divine Wisdom) by the Gnostics, is the ambassador to Poets and Alchemists, Artists and Musicians from this other realm. Inspiring us to ever greater heights of Creation in the hope of Transformation of the "Real World" into a realm wherein the imagination becomes real and the real becomes the imagination.
Yeah, I know it's a little paradoxical. But if you "muse" upon this for a while

I guarantee it will all make sense. Your intellectual instincts are generally unerring Otto's World.
Gandalfs BeardP.S. McKenna speculates on the nature of the mechanism as you describe -- but he also delves into notions of the Shamanistic ingestion of psychotrropic substances and the role of media, traditional (art, poetry, music, books) and electronic (TV, the Internet) as a kind of Feedback loop that is both inspired by and inspires The Muse.