One issue with trying to look into entity contact on mescaline is Carlos Castenada's claim of experiencing the entity 'Mescalito'. Castenada's work is supposedly all made up but others have still gone on to report contact with this entity e.g. D.M. Turner in The Little Book of Ketamine.
With regards to imagination being the cause of contact on tryptamines, it cannot be entirely ruled out. Cultures across the planet that supposedly did not come into contact with each other end up building pyramids for example. Or develop similar philosophical/metaphysical ideas. This could possibly be coming from common archetypes of the human mind? There is lots we still don't know about how hallucinogens work on the mind, or even how these receptors fully affect us day to day when not under the influence. I've not read Supernatural so I don't know if this was covered at all.