gistfile1.php

<?php

add_filter('query_vars', function($vars){
  $vars[] = 'featured';
	return $vars;

});

add_action('init', function(){
	add_rewrite_rule('sponsors/test/?$', 'index.php?post_type=sponsors&featured=true', 'top');
});

add_action('wp_head', function(){
	echo "featured is ... ";
	echo get_query_var('featured') ? get_query_var('featured') : ' NOT SET!';  
});