Allow choosing of a custom menu and saving that value as postmeta

custom-menu-metabox-field.php

<?php

/**
 * Show custom menus as select list
 * For use with CMB for WP: https://github.com/jaredatch/Custom-Metaboxes-and-Fields-for-WordPress/wiki/Adding-your-own-field-types
 * @author Travis Northcutt
 * @link https://gist.github.com/2284508
 *
 * @param array $field
 * @param string $meta
 * 
 */
add_action( 'cmb_render_custom_menu', 'ba_cmb_render_custom_menu', 10, 2 );
function ba_cmb_render_custom_menu( $field, $meta ) {
	$menus = get_terms( 'nav_menu', array( 'hide_empty' => false ) );
	if ( !$menus ) {
		echo '<p>'. sprintf( __('No menus have been created yet. <a href="%s">Create some</a>.'), admin_url('nav-menus.php') ) .'</p>';
		return;
	}
	
	echo '<select name="', $field['id'], '" id="', $field['id'], '">';
	echo '<option value="">None</option>';
	foreach ( $menus as $menu ) {
		echo '<option value="', $menu->term_id, '"', $meta == $menu->term_id ? ' selected="selected"' : '', '>', $menu->name, '</option>';
	}
	echo '</select>';
	echo '<p class="cmb_metabox_description">', $field['desc'], '</p>';
}

/**
 * Add filter to make the newly defined custom_menu meta field work with CMB for WP
 * forked from Travis Northcutt's gist: https://gist.github.com/2284508
 * @author Pat Ramsey
 * 
 * 'type' is set to the latter part of the cmb_render_ action created above. In this case 'custom_menu'
 * 
 */

add_filter( 'cmb_meta_boxes', 's25_menu_meta_boxes' );
function s25_menu_meta_boxes( $meta_boxes ) {
	$meta_boxes[] = array (
		'id' => 's25_menu_box',
		'title' => 'Custom Menu',
		'pages' => array ( 'page' ),
		'context' => 'normal',
		'priority' => 'high',
		'show_names' => true, // Show field names on the left
		'fields' => array (
			array (
				'name' => 'Custom Menu',
				'id' => 's25_custom_menu',
				'type' => 'custom_menu',
				'desc' => 'Invalid email addresses will be wiped out.'
			)
		)
	);
	return $meta_boxes;
}