Capturing keyup event (JavaScript) from Drupal editors (CKeditor)
Well, once again I've come up with my own answer, though it's not really the one I wanted.

First off, it does not appear that there is any Drupal JavaScript API for interacting with editors. So I had to go directly to the CKEditor API to capture the key up event.

This is the code I was eventually able to use:

CKEDITOR.on('instanceReady', function () {
  // Get the HTML ID of the textarea input that would be used if the editor were not enabled:
  var fieldID = $(".some_textarea").attr("id");
  if (CKEDITOR.instances[fieldID]) {
    // Add keyup listener.
    CKEDITOR.instances[fieldID].on("key", function () {
      // The last key pressed isn't available in editor.getData() when
      // the key is pressed. A workaround is to use setTimeout(), with no
      // time set to it, as this moves it to the end of the process queue,
      // when the last pressed key will be available.
      var editor = this;
      window.setTimeout(function () {
        var text = editor.getData();
        // Do something with the text.
        // Also note that the original textarea can be
        // retrieved from editor.element.$. So you could
        // do this:
        var originalTextarea = editor.element.$;
      });
    });
  }
});