Best practice – Meta Query vs. post_clauses for “left join” ordering

You can use pre_get_posts with a callback:

<?php
defined( 'ABSPATH' ) OR exit;
/** Plugin Name: (#102854) Order Posts by Foo */

add_filter( 'pre_get_posts', 'wpse_102854_orderby_foo' );
function wpse_102854_orderby_foo( $query )
{
    if ( 
        ! $query->is_main_query()
        OR ! is_admin()
        OR 'edit.php?post_type=YOUR_POST_TYPE' !== $GLOBALS['parent_file']
        // Other conditions that force an abort, Example:
        // OR 'foo' !== $query->get( 'some_query_var' )
    )
        return $query;

    $query->set( 'orderby', 'meta_value_num' );
    // etc.

    return $query;
}