^^I've heard this theory of Mazatec use as well, but I've not encountered a detailed ethnobotanical description of use, and thus am suspicious. I've also read/heard that rumours of Mazatec use of Coleus may be untrue, and that the plant is simply grown for its foliage, and there is no actual shamanic use of it, and this been misinterpreted at some point.
An interesting thing worth mentioning...both Salvia divinorum and Psilocybe mexicana are native species to the area of Mexico the Mazatec inhabit, and have likely been used ritually by them for a long time. The origin of Coleus blumei is Southeast Asia and Malaysia. When shamans employ plants and fungi, they typically employ those species native to the particular ecosystems they inhabit. Thus I think that Coleus has made a much more recent appearance there. Psilocybe cubensis I believe was originally of Asian origin, and while the Mazatec do employ this species, it considered the least desirable species to work with (an introduced, weed mushroom that grows on dung, better off sold to tourists), apparently Mazatec shaman Maria Sabina refused to work with it. Whatever the case, it is interesting as there are quite a few Erowid Coleus reports, describing activity.