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Could XCE implement locating rules, the same as nxml-mode uses? It's basically just an XML file that tells you how to locate the corresponding schema file. So, for example, this rule would search for a RELAX NG compact schema in the same directory as the main file:
<transformURI pathSuffix=".xml" replacePathSuffix=".rnc"/>
So, for example, for "example.xml" the schema to use would be "example.rnc". You get the point.
I haven't found any official docs yet, but there is an example file here: http://infohost.nmt.edu/tcc/help/pubs/nxml/default-schemas.html