If you map the locations of 20s scenarios in English-language publications, I think you'll find a clear predominance of scenarios set either in English-speaking countries or in countries in which English or American colonial interests were focused. This comes both from the visibility of pop-culture impressions of the place in English media and the availability of accessible and detailed period research material on the subject in English. French Indochina therefore has two strikes against it right in the name.
I'm thinking the "anglo-centric" explanation doesn't really fly, considering the number of scenarios and even a source book set in Central and South America. While one of those scenarios was set in a British colony (British Honduras -- modern Belize), the rest have been set in places like Mexico, Peru, Paraguay and Brazil. Which, while possibly influenced by outside societies, were more influenced by Portuguese and Spanish society (and even, to some extent by German society -- in the case of Paraguay, for example). Egypt has received tons of attention over the years, with dozens of scenarios set there, the Cairo Guidebook, and so on, and while you could argue that it was a British colonial interest, it was also heavily influenced by the French and for much the same reason the British were interested. In fact, the French are the ones who built the Suez Canal. But more importantly, the whole "Egypt" thing conjures up a certain mind-set of mystery (at least partially due to The Raiders of the Lost Ark, I suspect) in a lot of people. So no, I think it's just been an area that's been almost wholly neglected.
One possible explanation is that there just isn't a lot of popular writing or movie making on the SE Asia area these days (other than the occasional Vietnam War pic), so people tend not to automatically think of the jungles of Burma or Thailand or Indochina as Places of Mystery. If you go back and dig into the stuff being written back in the 1890s-1920's there WAS some good stuff (the Dacoits, if I recall correctly, were primarily in Burma and parts of modern Bangladesh and were one of the groups used by Fu Manchu as assassins), but as World War II got closer the focus tended to shift away from there and out into the Pacific basin or China more.
In a way, it's like the "monster du jour" effect that we've seen crop up from time to time. There for a while practically every scenario featured the Deep Ones as the baddies. Lately it seems like Hastur is the primary focus of lots and lots of stuff. Maybe we just need to break the "SE Asia barrier" and lots of good adventures will start to pop out of the jungle.
Lammomedes, one thing that MIGHT be helpful to you, is the Trail of Cthulhu source book Mythos Expeditions. While it has rules for how to conduct expeditions (which frankly seem a bit "clunky" to me; I'm kind of surprised actually since normally Ken Hite does a pretty good job of keeping the rules smooth), it also discusses quite a few out of the way places -- including Papua-New Guinea. While that's off the beaten path you're looking for it might help somewhat with the SE Asia style of jungle (which does differ from it's African and South American counterparts). Finally, I did find a brief discussion of the Khmer Empire, Angkor Wat, Angkor Thom and some discussion of travel in the Cambodian jungle in GURPS Classic: Places of Mystery (pp 58-9). I'll keep looking, but it's looking unlikely right now.