I know you could potentially use the WebDav features to be able to open the file with some other program, but I have not seen a way to edit directly in Kentico. Since the Portal Engine requires some kind of Kentico component to be on the page, I am not sure how you would be able edit just an HTML page.
I suppose you could write some kind of custom control to edit the HTML inline by loading it into a WYSIWYG type interface and re-saving it back into the HTML file.