I thought CoconJS would support XML one day, but I'm wrong. CocoonJS and Ejecta do not support XML parsing today.
It seems that JSONImporter plugin might be more general than TMXImporter plugin,
I am consider that to re-make another kind of tmx importer plugin, to split the interface and parsers. i.e.
tmx reader plugin + ( tmxXMLparser / tmxJSONparser plugin)