Hi there - great that you're working on this.
First, if you want to go back to the original method, i.e. using the 'html' format in the database, you can make your viewer take precedence over qa_viewer_basic by returning a value greater than 1.0 from the calc_quality(...) function in your viewer's class. Of course you should only do this if the $format parameter you're supplied is 'html'.
Hope this helps - feel free to ask any more questions about this.