I like the addition of the font stack to the mce editor quite a bit. However, I had to modify the code to not overwrite the URL to my theme's editor-style.css file when updating content_css. In the extend_tinymce_dropdown function I changed:
$opt['content_css'] = $content_css;
$opt['content_css'] .= ',' .$content_css; // modify to add to content_css with a comma, rather than replace all content - jv
This change or something like it would help your plugin integrate easier with a wider range of WP configurations.