There is an article that goes pretty indepth in respect to the SAR of mitragynine and its analogs, and does include dissociation constant data i believe. I think i shared it in the NPD subforum at some point.
Not sure if its correct but I get the sense that many of the alkaloids, at least the opioidergic ones, are mostly on the gradient of lowish efficacy partial agonists. They don't seem to precipitate withdrawal and those so called antagonists are most likely very low efficacy agonist, meaning that they probably activate the opioid receptors somewhat. Aside from MOR partial agonism they probably slightly activate the DOR and KOR as well, depending on the alkaloid.
You have to think of receptor activation and occupation without activation (ie, antagonism) as a sort of spectrum rather than just agonist versus antagonism.