[XeTeX] OpenType <ccmp> feature not working in XeLaTeX

[David Perry]

Dear All,

I've created a font that includes access to some ligatures needed by 
scholars, using the <ccmp> feature and the zero-width joiner (U+200D). 
For instance, if a user enters 'A ZWJ V' an A_V ligature (used by 
numismatists) is displayed.  The inclusion of ZWJ in the sequence 
prevents unwanted replacements, as in the word AVERY.

The font works fine in FontLab's OT preview and in Mac OS (three word 
processors tested).  But in XeLaTeX, both on Windows and Mac, the 
ligatures defined in <ccmp> don't work (the font works OK otherwise). 
Does XeTeX not support this?  Any workarounds?

Thanks - David