A reader of this blog has pointed out to me that, apparently without any notice, Apple has expanded OS X support for OpenType font features, so that at least some apps can now use Windows fonts as well as the AAT fonts supplied by Apple for complex Indic scripts like Devanagari. A good test is to use the font Sanskrit 2003 in TextEdit.
This is a major improvement, because it opens up a much larger range of fonts than have been possible up to now for scripts used by a major segment of the world population. Unfortunately my tests (so far only with Devanagari) indicate that this capability is not yet available in iWork apps or Openoffice or Mellel. But it does work in Nisus Writer.
I understand that this new feature should be available in any apps that use Core Text.