Hello, we're using your plugin for a site that has a fairly large list of users.
One bug we've run across is where the function wp_dropdown_users times out because of the number users it's trying to request (in the thousands).
In our case 99% of the users are subscribers, WordPress added an option to wp_dropdown_users called 'who' with only one option which is 'authors' what this does is filteres out all users who can't author a post from the dropdown. This fixes our problem and I doubt that you would be using WordPress SEO to promote your Subscriber role authors.
Is it possible to add this option to where you use the function on the Meat Titles options page?
Your new function call would look like this.
wp_dropdown_users( array( 'show_option_none' => "Don't show", 'name' => 'wpseo_titles[plus-author]', 'class' => 'select', 'selected' => isset( $options[ 'plus-author' ] ) ? $options[ 'plus-author' ] : '', 'who' => 'authors' ) );