Erwin has posted my updated version of CompositeData.xml for those who need to complete composites for the Extended Greek character set. Make a backup copy of the existing CompositeData.xml file, and save this update to your <install directory\Composites\ folder.
Your font will need to contain the basic Greek characters in the Greek and Coptic character set. The diacritics for extended Greek, which are required for the compose composites feature to work, can themselves be composed from spacing modifier accents, diaeresis, grave, acute, dot accent, and right single quotation mark. Accent Prosgegrammeni must be created as a simple glyph.
It is recommended to adjust the position of diacritics for lowercase alpha, before manually adjusting the position for other characters using snap to guidelines. As always with this feature, the precise position is only correct for the font used while creating the data file. All other fonts will require manual adjustment of diacritics, and entry of some font metrics.
Extended Greek Character Set
Copy this data to the Insert Characters dialogue to add the entire Extended Greek Character set.
7936-7957,7960-7965,7968-8005,8008-8013,8016-8023,8025,8027,8029,8031,8032-8061,8064-8116,8118-8132,8134-8147,8150-8155,8157-8175,8178-8180,8182-8190
Please reply to this thread if the feature is not working as expected.