您可以尝试添加geo_country 作为额外的查询变量，具有：

/**
 * Add the \'geo_country\' as a public query variable
 *
 * @param array $query_vars
 * @return array $query_vars
 */ 
function my_query_vars( $query_vars ) 
{
    $query_vars[] = \'geo_country\';
    return $query_vars;
}

add_filter( \'query_vars\', \'my_query_vars\' );

/**
 * Filter the feed by the \'geo_country\' meta key 
 *
 * @param WP_Query object $query
 * @return void 
 */ 
function my_pre_get_posts( $query ) 
{
    // only for feeds
    if( $query->is_feed && $query->is_main_query() ) 
    {
        // check if the geo_country variable is set 
        if( isset( $query->query_vars[\'geo_country\'] ) 
                && ! empty( $query->query_vars[\'geo_country\'] ) )
        {

            // if you only want to allow \'alpha-numerics\':
            $geo_country =  preg_replace( "/[^a-zA-Z0-9]/", "", $query->query_vars[\'geo_country\'] ); 

            // set up the meta query for geo_country
            $query->set( \'meta_key\', \'geo_country\' );
            $query->set( \'meta_value\', $geo_country );
        }

    } 
}

add_action( \'pre_get_posts\', \'my_pre_get_posts\' );

这适用于我的安装，主题为“二十一二”。