Have you (or do you know of anyone) who has developed a plugin for oXygen which allows editors to search through external XML files (or an xml database) from oXygen's autocomplete / content completion interface?
An example of how such a plugin might be used is in doing personography work with primary source materials, and at the point in the TEI editing process where you add the @ref attribute to a <persName> tag, a searchable pop-up list appears, populated with entries from your personography file.
Such a plugin wouldn’t necessarily modify oXygen’s content completion functionality, but I imagine it would somewhat mimic its behavior.
Digital Humanities Librarian
River Campus Libraries, University of Rochester