Hm, I tried
IEditorReference references = page.findEditors(input, null, IWorkbenchPage.MATCH_INPUT);
Unfortunately the returned list is always empty. I'm using the EditorInput passed on many other places, so I do not believe my input is wrong. When debugging into the method getEditorReferences() returns the empty list. This method calls getEditors from EditorAreaHelper, which calls EditorSashContainer#getEditorWorkbooks(). This method returns one EditorStack object with the id 'DefaultEditorWorkbook'. Its children list is empty.