import-meta-backup.php

<?php

/*
* Plugin Name: Import Meta Backup
* Plugin URI: http://somesite.com/where-this-plugin-lives/
* Description: Backs up original data in meta.
* Author: Automattic
* Author URI: http://theauthor.com/
* Version: 1.0
* Text Domain: import-meta-backup
* License: GPLv3 or later
*/

class Import_Meta_Backup {
    function __construct() {
        // Set the import origin from -origin=someorigin
        $args= $_SERVER['argv'];

        foreach( $args as $arg ) {
            if( false !== strpos( $arg, '-origin=' ) ) {
                $this->origin = str_replace( '-origin=', '', $arg );
                break;
            }
        }

        add_filter( 'wp_import_post_data_raw', function( $post ) {
            if( ! isset( $post['postmeta'] ) )
                $post['postmeta'] = array();

            $post['postmeta'][] = array(
                'key' => '_original_post_id',
                'value' => $post['post_id'],
            );

            $post['postmeta'][] = array(
                'key' => '_original_import_author',
                'value' => $post['post_author'],
            );

            if ( ! empty( $post['post_parent'] ) ) {
                $original_post_parent = $post['post_parent'];
                $post['postmeta'][] = array(
                    'key' => '_original_parent_id',
                    'value' => $post['post_parent'],
                );
            }

            if( ! empty( $this->origin ) ) {
                $post['postmeta'][] = array(
                    'key' => '_original_import_origin',
                    'value' => $this->origin,
                );
            }

            foreach( $post['postmeta'] as $meta ) {
                if ( $meta['key'] == '_thumbnail_id' ) {
                    $post['postmeta'][] = array(
                        'key' => '_original_thumbnail_id',
                        'value' => $meta['value'],
                    );
                    break;
                }
            }

            if ( 'attachment' == $post['post_type'] ) {
                $original_attachment_url = ! empty( $post['attachment_url'] ) ? $post['attachment_url'] : $post['guid'];
                $post['postmeta'][] = array(
                    'key' => '_original_import_url',
                    'value' => $original_attachment_url,
                );
            }

            return $post;
        });
    }
}

new Import_Meta_Backup;