Nothing of the sort is available on Macintosh (partly because Mac applications that support ATSUI appear to be even more rare;-().
Unicode support in applications is widespread on Mac OS X. In particular, all the built-in applications should be able to handle Khmer with a properly constructed font. The system already supports Thai out of the box, via Unicode. It's true that neither a font nor a keyboard for Khmer are included with the OS.
Deborah Goldsmith Manager, Fonts / Unicode liaison Apple Computer, Inc. [EMAIL PROTECTED]