PageTemplater

<?php
/*
Plugin Name: PageTemplater
Description: Allows page templates in plugin/templates/ folder to appear and be used on page edit screen. Modified by bcworkz to work on quick edit form.
Author: Harri Bell-Thomas
Version: 0.50 beta
License: GPL
*/

class PageTemplater {

        /**
         * A Unique Identifier
         */
         protected $plugin_slug;

        /**
         * A reference to an instance of this class.
         */
        private static $instance;

        /**
         * The array of templates that this plugin tracks.
         */
        protected $templates;

        /**
         * Returns an instance of this class.
         */
        public static function get_instance() {

                if( null == self::$instance ) {
                        self::$instance = new PageTemplater();
                }

                return self::$instance;

        }

        /**
         * Initializes the plugin by setting filters and administration functions.
         */
        private function __construct() {

                $this->templates = array();

                // Add a filter to the attributes metabox to inject template into the cache.
                // --Does nothing for page attributes, this filter fires just before the cache is called
                //   by page_template_dropdown()
                add_filter(
                    //'page_attributes_dropdown_pages_args',
                    // 'wp_dropdown_pages' fires for both quick edit and edit pages, the attributes
                    //      filter only fires for edit pages -- bcworkz
                    'wp_dropdown_pages',
                     array( $this, 'register_project_templates' )
                );

                // Add a filter to the save post to inject out template into the page cache
                add_filter(
                    'wp_insert_post_data',
                    array( $this, 'register_project_templates' )
                );

                // Add a filter to the template include to determine if the page has our
                // template assigned and return it's path
                // --Loads template when page dependent on it is displayed
                add_filter(
                    'template_include',
                    array( $this, 'view_project_template')
                );

                // Add your templates to this array.
                $this->templates = array(
            'mytemplate-test.php'     => 'Injected Template',
        );
}

        /**
         * Adds our template to the pages cache in order to trick WordPress
         * into thinking the template file exists where it doens't really exist.
         *
         */

        public function register_project_templates( $atts ) {

                // Create the key used for the themes cache
                $cache_key = 'page_templates-' . md5( get_theme_root() . '/' . get_stylesheet() );

                // Retrieve the cache list.
                // If it doesn't exist, or it's empty prepare an array
                $templates = wp_get_theme()->get_page_templates();
                if ( empty( $templates ) ) {
                        $templates = array();
                }

                // New cache, therefore remove the old one
                wp_cache_delete( $cache_key , 'themes');

                // Now add our template to the list of templates by merging our templates
                // with the existing templates array from the cache.
                $templates = array_merge( $templates, $this->templates );

                // Add the modified cache to allow WordPress to pick it up for listing
                // available templates
                wp_cache_add( $cache_key, $templates, 'themes', 1800 );

                return $atts;

        }

        /**
         * Checks if the template is assigned to the page
         */
        public function view_project_template( $template ) {

                global $post;

                if (!isset($this->templates[get_post_meta(
                    $post->ID, '_wp_page_template', true
                )] ) ) {

                        return $template;

                }

                $file = plugin_dir_path(__FILE__). 'templates/' .get_post_meta(
                    $post->ID, '_wp_page_template', true
                );

                // Just to be safe, we check if the file exist first
                if( file_exists( $file ) ) {
                        return $file;
                }
                else { echo $file.' not found!'; }

                return $template;

        }

}

add_action( 'plugins_loaded', array( 'PageTemplater', 'get_instance' ) );