WordPress: Make Piklist plugin show metabox only for top-level pages (i.e., post_parent = 0)

wp-piklist-metabox-top-pages-only.php

<?php
/**
 * This code went into the file in piklist/parts/metaboxes
 */

/*
Title: Section Name
Post Type: page
Top pages only: true
*/


/**
 *  These two functions went into my functions.php
 */
 
add_filter( 'piklist_get_file_data', 'mcw_custom_comment_block', 10, 2 );
function mcw_custom_comment_block ( $data, $type ) {
	// if not a meta box file then bail
	if ( $type != 'meta-boxes' ) {
		return $data;
	}

	// allow Piklist to read our custom comment block attributes
	$data['top_pages_only'] = 'Top pages only';
	return $data;
}


add_filter( 'piklist_add_part', 'mcw_check_custom_comment_block', 10, 2 );
function mcw_check_custom_comment_block ( $data, $type ) {
	global $post;

	// if not a meta box then bail
	if ( $type != 'meta-boxes' ) {
		return $data;
	}

	// check if any custom requirements are set in the comment block
	if ( !empty( $data['top_pages_only'] ) ) {
		// is this a top-level page?
		if ( $post->post_parent ) {
			// change meta-box access to user role: no-role
			$data['role'] = 'no-role'; 
		}
	}
	return $data;
}
?>