Add Standard Page Attributes Metabox for Page Parent

Although not listed in the following resources, seems that parent_id is a reserved name in $_POST and $_REQUEST:

• Function_Reference/register_taxonomy#Reserved_Terms

• List of reserved names in $_POST and $_REQUEST used by WordPress 3.5

This code works in a Woocommerce product page to show a dropdown of regular pages. Also, in your code, you were missing the get_post_meta to fill the selected argument value.

add_action( 'add_meta_boxes','add_metabox_wpse_83542' );
add_action( 'save_post', 'save_post_wpse_83542', 10, 2 );

function add_metabox_wpse_83542() 
{
    add_meta_box(
        'postparentdiv', 
        __('Page Parent'), 
        'meta_box_content_wpse_83542', 
        'product', 
        'side', 
        'core'
    );
}

function meta_box_content_wpse_83542( $post ) 
{
    $meta = get_post_meta( $post->ID, '_parent_id', true );
    $selected = ( isset( $meta ) ) ? $meta : '';
    $dropdown_args = array(
        'post_type'        => 'page',
        'exclude_tree'     => $post->ID,
        'selected'         => $selected,
        'name'             => '_parent_id',
        'show_option_none' => __( '(no parent)' ),
        'sort_column'      => 'menu_order, post_title',
        'echo'             => 0,
        'child_of'         => 97, // Sales Page
    );

    $dropdown_args = apply_filters( 'page_attributes_dropdown_pages_args', $dropdown_args, $post );
    $pages = wp_dropdown_pages( $dropdown_args );

    if ( ! empty($pages) ) 
    {
        wp_nonce_field( plugin_basename( __FILE__ ), 'noncename_wpse_83542' );          
        ?>
        <p><strong><?php _e('Parent') ?></strong></p>
        <label class="screen-reader-text" for="parent_id"><?php _e('Parent') ?></label>
        <?php 
        echo $pages; 
    } // end empty pages check
}    

function save_post_wpse_83542( $post_id, $post ) 
{
    // Block on Autosave
    if ( defined( 'DOING_AUTOSAVE' ) && DOING_AUTOSAVE )  
        return;

    // Block Revisions
    if ( 'revision' == $post->post_type )
        return;

    // Nonce verify
    if ( !wp_verify_nonce( $_POST['noncename_wpse_83542'], plugin_basename( __FILE__ ) ) )
        return;

    if ( $post->post_type == 'product' && !empty( $_POST['_parent_id'] ) ) 
    {
        update_post_meta( $post->ID, '_parent_id', $_POST['_parent_id'] );
    }
}