The various threads on Tantric Theravada may be of interest in terms of the background of the various influences that exist in SE Asian Buddhism: http://www.dhammawheel.com/viewtopic.ph ... 03&start=0http://www.dhammawheel.com/viewtopic.php?f=13&t=6599http://www.dhammawheel.com/viewtopic.php?f=24&t=833
I recall Ven Gavesako, and others, posting interesting links to discussions on how the various folk influences, along with some Mahayana influences, were suppressed in SE Asia in the 19th and early 20th Centuries, e.g. with the establishment of the Dhammayuttika monastic order by King Monkut. From that point of view, it's not so much that the various Buddhist strands have recently infiltrated SE Asia, but that they were suppressed for a time.
I'm sure Ven Gavesako and Tilt can dig out some of the specific references I'm thinking of, since, as you say, we've discussed before these possible influences on the Forest Ajahns, such as Ajahn Mun.