According to an excerpt from an interview with Peter Weir, a French guy who actually done the walk told him about the bark necklace, doesn't say what it is, but might narrow it down a bit?
Peter Weir: "Oh yes, I was always changing things as a result of that. Oh, a third person Cyril Delafosse-Guiramand, late 30s, French. He works in the IT industry but his hobby is walking. He was inspired by the book and sometime after 2000 he undertook the walk, so he was the first person I went to see. He was living in Laos then with his wife. So I went up there and we talked for a couple of days and I said, 'Would you advise us? d Ill send you scripts.' Then I began to say, 'Can I put some of the stuff you're telling me in, like this thing about these mosquitoes? This man you met in the forest that has a bark necklace?' He said, 'Sure,' very generously so I put that in.. Then he coached the actors and then was with us every day."
Lemon Eucalyptus for example is as good as or better than DEET for repelling the lil bleeders, seems alot of citrus type plants are, citronella, lemon balm, lemon sage, basil. Not sure what trees would have been in that geographic though?
Si