I know this is very late, but I have found out what bai thawn laang (mentioned on the previous page re. the Miang of Pomelo) are - they are not Betel leaves, but leaves from the "Tiger's Claw" or "Indian Coral" tree. Bai means "leaf", thawn means "gold(en)", and laang means "omen" - that in itself didn't help, but when I searched on the Thai Royal Botanical Society website with the name written in Thai, I found it pretty easily... A quick trip to Wikipedia with the Latin name gave me this:
"In Vietnam, the leaves are used to wrap fermented meat (Vietnamese: nem)." I guess if they're used in Vietnam it'd make sense for them to be found in Thailand too... Hope that's useful, or least interesting!
EDIT: Here's the whole genus page - https://en.wikipedia.org/wiki/Erythrina#Use_by_humans