When in a modal dialog, the parent window has an object called "app" which is of type MT.App. This object in turn has a property called "editor" which is of type Editor. You can find the files for these objects under mt-static/js/common. From the modal dialog, you can access them with calls like this:

<script type="text/javascript">
    window.parent.app.insertHTML("<strong>I just inserted text!</strong>");
    alert("Now things are about to be reset!");
    window.parent.app.editor.setHTML("");
    //Now everything should be cleared out of the editor.
</script>

The MT.App class has an insertHTML method which is a thin wrapper around the one provided by Editor. Editor has two other methods, get and setHTML, which are of value for developers who want to manipulate the contents of the text editor.