Customize Queried Post Info plugin: Communicate the singular post queried object currently previewed in the Customizer.

class-customize-queried-post-info.php

<?php
/**
 * Customize_Queried_Post_Info class.
 *
 * @package CustomizeQueriedPostInfo
 */

/**
 * Class Customize_Queried_Post_Info.
 */
class Customize_Queried_Post_Info {

	/**
	 * Add hooks for plugin.
	 */
	public function init() {
		add_action( 'customize_preview_init', array( $this, 'customize_preview_init' ) );
		add_action( 'customize_controls_enqueue_scripts', array( $this, 'enqueue_control_scripts' ) );
	}

	/**
	 * Enqueue Customizer control scripts.
	 */
	public function enqueue_control_scripts() {
		$handle = 'customize-post-previewed-controls';
		$src = plugins_url( 'customize-controls.js', __FILE__ );
		$deps = array( 'customize-controls' );
		$ver = false;
		$in_footer = true;
		wp_enqueue_script( $handle, $src, $deps, $ver, $in_footer );
	}

	/**
	 * Initialize Customizer preview.
	 */
	public function customize_preview_init() {
		add_action( 'wp_enqueue_scripts', array( $this, 'enqueue_preview_scripts' ) );
	}

	/**
	 * Enqueue script for Customizer preview.
	 */
	public function enqueue_preview_scripts() {
		$handle = 'customize-post-previewed-preview';
		$src = plugins_url( 'customize-preview.js', __FILE__ );
		$deps = array( 'customize-preview' );
		$ver = false;
		$in_footer = true;
		wp_enqueue_script( $handle, $src, $deps, $ver, $in_footer );

		$wp_scripts = wp_scripts();
		$queried_post = null;
		if ( is_singular() && get_queried_object() ) {
			$queried_post = get_queried_object();
			$queried_post->meta = get_post_custom( $queried_post->id );
		}
		$wp_scripts->add_data( $handle, 'data', sprintf( 'var _customizePostPreviewedQueriedObject = %s;', wp_json_encode( $queried_post ) ) );
	}
}

customize-controls.js

/* global wp, console */
( function( api ) {

	var self;

	self = {
		queriedPost: new api.Value()
	};

	/**
	 * Listen for queried-post messages from the preview.
	 */
	api.bind( 'ready', function() {
		api.previewer.bind( 'queried-post', function( queriedPost ) {
			self.queriedPost.set( queriedPost || false ); /* Note that Value.set ignores null, hence false here. */
		} );
	} );

	/**
	 * Example plugin code that can now listen for when a post is being previewed or ceases to be previewed.
	 */
	self.queriedPost.bind( function( newPost, oldPost ) {
		if ( newPost ) {
			if ( oldPost ) {
				console.info( 'You are no longer previewing: '  + oldPost.post_title + ' (post ID: ' + oldPost.ID + ')' );
			}
			console.info( 'You are now previewing: '  + newPost.post_title + ' (post ID: ' + newPost.ID + ')' );
		} else {
			console.info( 'You are no longer previewing a singular post template.' );
		}
	} );

} )( wp.customize );

customize-preview.js

/* global wp, _customizePostPreviewedQueriedObject */
( function( api ) {

	var self;

	self = {
		queriedPost: null
	};
	if ( 'undefined' !== typeof _customizePostPreviewedQueriedObject ) {
		self.queriedPost = _customizePostPreviewedQueriedObject;
	}

	/**
	 * Send the queried post object to the Customizer pane when ready.
	 */
	api.bind( 'preview-ready', function() {
		api.preview.bind( 'active', function() {
			api.preview.send( 'queried-post', self.queriedPost );
		} );
	} );

} )( wp.customize );

customize-queried-post-info.php

<?php
/**
 * Plugin Name: Customize Queried Post Info
 * Plugin URI: https://gist.github.com/westonruter/1feb78845c8e562dd59d
 * Description: Communicate the singular post queried object currently previewed in the Customizer.
 * Version: 0.1
 * Author:  XWP, Weston Ruter
 * Author URI: https://xwp.co/
 * License: GPLv2+
 * License URI: http://www.gnu.org/licenses/gpl-2.0.txt
 *
 * Copyright (c) 2015 XWP (https://xwp.co/)
 *
 * This program is free software; you can redistribute it and/or modify
 * it under the terms of the GNU General Public License, version 2 or, at
 * your discretion, any later version, as published by the Free
 * Software Foundation.
 *
 * This program is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
 * GNU General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License
 * along with this program; if not, write to the Free Software
 * Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA
 *
 * @package CustomizeQueriedPostInfo
 */

require_once __DIR__ . '/class-customize-queried-post-info.php';
$customize_queried_post_info = new Customize_Queried_Post_Info();
$customize_queried_post_info->init();