Definitely file this one under "apocryphal", but it does sound like a possible viable approach to sublingual bufo dosing. I also suspect that the "green phlegm" drawn from the protagonist's body was likely coloured with a chewed up leaf of some kind, concealed in the shaman's cheek. The cardiotoxic components of toad venom - making a huge assumption here about the actual toad species, of course - with names along the lines of bufadienolides and the like, would be reasonably heat stable but the addition of warm saliva, and possibly the suspected leaf juice admixture, might serve to lop off the saccharide portion of these toxins through the action of salivary amylase and render them much more benign.

Just an educated guess but, as you say, "more information needed" - it would probably be worth contacting the author. He does appear to have a large selection of tall tales that stretch ones credulity...