Put this file on your mu-plugins WordPress directory, and map your custom URL, title, meta-description as a CSV file. Used i.e http://rubbettinoprint.it/

Universal SEO mu-plugin.php

<?php

/**
 *
 * Plugin Name: Universal SEO Plugin
 * Description: Set and overwrite title/meta description via url, i.e mapped in a .txt file
 * Author: Salvatore Capolupo
 * Version: 1.0
 * Author URI: http://www.rubbettinoprint.it
 */
  
/**
seodata.txt example
----
# this is a comment, please use ; as a separator
# format: URL;title;description;

# home 
/;Home title;Home description;
 
#...
#...
----
**/
  
class SEOUtility{
	var $data;
	var $csv_sep = ";";
	var $sep = " * ";
	var $nodata = "-";
	
	function init(){
		$basepath = plugin_dir_path( __FILE__ );
		$file = fopen( $basepath . "seodata.txt", "r");//or exit("seodata.txt non presente o non leggibile.");
		while( !feof($file) ){
			$riga = fgets($file);
			if ( !preg_match("/\#/", $riga) ){
				$riga = explode(  $this->csv_sep, $riga );
				$this->data[ $riga[0] ] = $riga[1] . $this->sep .$riga[2];
			}
  		}
		fclose($file);
	}

	public function get_current_url( ){
		return "$_SERVER[REQUEST_URI]";
	}

	public function was_set_data_for( $url ){
		return array_key_exists( $url, $this->data);
	}

	public function get_title_for( $url ){
		if ( !array_key_exists( $url, $this->data) ) return $this->nodata;
		
		$tmp = $this->data[$url];
		$riga = explode(  $this->sep, $tmp);
		
		return $riga[0];
	}

	public function get_description_for( $url ){
		if ( !array_key_exists( $url, $this->data) ) return $this->nodata;

		$tmp = $this->data[$url];
		$riga = explode(  $this->sep, $tmp);
		
		return $riga[1];
	}

}

//abilita supporto ai title, se non presente
function theme_slug_setup() {
   add_theme_support( 'title-tag' );
}
add_action( 'after_setup_theme', 'theme_slug_setup' );

//title rewrite
add_filter('document_title_parts', 'rp_override_post_title', 1);
function rp_override_post_title($title){
	$SEOobj = new SEOUtility();
	$SEOobj -> init(); 

	if ( $SEOobj->was_set_data_for( $SEOobj->get_current_url() ) )
		$title['title'] = $SEOobj->get_title_for( $SEOobj->get_current_url() ) . " - ". get_bloginfo('name');
    
    return $title; 
}

// add meta-description
add_action( 'wp_head', 'cyb_author_archive_meta_desc' );
function cyb_author_archive_meta_desc() {
	$SEOobj = new SEOUtilityCapolupo();
	$SEOobj -> init();

	if ( $SEOobj->was_set_data_for( $SEOobj->get_current_url() ) )
    	echo '<meta name="description" content="' . $SEOobj->get_description_for( $SEOobj->get_current_url() ) . '">';
}
?>