Bthoven,
Please, have a look at this Thai book at my PB301. Sorry for quick iPhone pictures.
All I did was copying one of the True Type Fonts, supporting Thai, Angsa.ttf, to the /system/fonts on the device memory. After that I opened the book and changed the setting to this font and auto encoding. Voila, it works!
Here is the link to a bunch of unicode Thai fonts:
http://www.wazu.jp/gallery/Fonts_Thai.html
The shipping and and handling cost to Thailand is $60.