Template rendering techniques used at https://selv.in/blog/rendering-templates-in-custom-locations

templaterendering.php

<?php
namespace Craft;

class MyPluginService extends BaseApplicationComponent
{
	const DEFAULT_TEMPLATES_PATH = 'path/to/templates/';

	/**
	 * Renders a template in a custom location defined by $path
	 *
	 * @param string $file
	 * @param array  $vars
	 * @param string $path
	 *
	 * @return \Twig_Markup
	 */
	public function renderCustomTemplate($file, array $vars = array(), $path = self::DEFAULT_TEMPLATES_PATH)
	{
		$oldPath = craft()->path->getTemplatesPath();

		craft()->path->setTemplatesPath($path);

		$html = craft()->templates->render($file, $vars);

		craft()->path->setTemplatesPath($oldPath);

		return TemplateHelper::getRaw($html);
	}

	/**
	 * Renders the content of a template file using absolute path defined by $file
	 *
	 * @param string $file
	 * @param array  $vars
	 *
	 * @throws Exception
	 * @return \Twig_Markup
	 */
	public function renderCustomTemplateString($file, array $vars = array())
	{
		if (is_readable($file))
		{
			$source   = file_get_contents($file);
			$rendered = craft()->templates->renderString($source, $vars);

			return TemplateHelper::getRaw($rendered);
		}

		throw new Exception(Craft::t('{f} is missing or cannot be read.', array('f' => $file)));
	}
}