The AppleScript uses a program called Cyclone to convert the source of a selected message in Entourage from Windows/DOS Thai to Unicode. It then creates a new note containing the converted message source, and links the note to the original message. I initially wrote the script so that it would overwrite the subject and content of the message with the converted text, but found this method to be destructive and unreliable (certain characters did not convert properly, for some reason).
Obviously, the script requires that Cyclone be installed. Also, the script should be placed in ~/Documents -> Microsoft User Data -> Entourage Script Menu Items, so that it will show up in the Script Menu within Entourage.
Something I discovered by accident (I don't think it's documented anywhere) is if you end the name of the script file with 'c' and some letter (mine is called "Convert Message from DOS Thai to UnicodecC"), you will be able to use Control-letter as a shortcut for the script in Entourage (in my case, Control-C). And of course, the script can be edited for conversion of other text encodings.
This hint is probably of use to only a small number of people, but I thought I'd submit it anyway, in case anyone has ideas about improvements or a better way to accomplish this task.

