Example of a class reflection to make the availability of this class optional.

Class_Reflector.php

<?php

/**
 * base reflector class to reflect plugins APIs without using
 * plugin classes directly
 *
 */
namespace Project\Theme\Reflector;

abstract class Class_Reflector {

	/**
	 * the name of the reflected class
	 *
	 * @var string
	 */
	protected $class = '';

	/**
	 * reference of the the object to reflect
	 *
	 * @var Instance of $class
	 */
	protected $object = NULL;

	/**
	 * reflector of the object
	 *
	 * @var ReflectionObject
	 */
	protected $reflector_object = NULL;

	/**
	 * gets the global $wpdb
	 *
	 * @param string $class Class to reflect
	 * @param mixed (passed to the constructor)
	 * @return DB
	 */
	public function __construct( $class ) {

		$args = array_reverse( func_get_args() );
		array_pop( $args );
		$args = array_reverse( $args );
		try {
			$this->reflector_object = new \ReflectionClass( $class );
			$this->object = $this->reflector_object->newInstanceArgs( $args );
		} catch( \ReflectionException $e ) {
			$error_level = \E_USER_NOTICE;
			if ( defined( '\WP_DEBUG' ) && \WP_DEBUG )
				$error_level = \E_USER_WARNING;
			$this->trigger_error( $e->getMessage(), $error_level );
		}
	}

	/**
	 * call methods of the object
	 *
	 * @param string $method
	 * @param array $args
	 * @return mixed
	 */
	public function __call( $name, $args = array() ) {

		if ( ! $this->object )
			return;

		$methods = $this->reflector_object->getMethods();
		foreach ( $methods as $m ) {
			if ( $name !== $m->getName() )
				continue;

			if ( $m->isPrivate() )
				return $this->trigger_error( 'Access to private method "' . $name . '" of ' . $this->class, \E_USER_ERROR );

			if ( $m->isProtected() )
				return $this->trigger_error( 'Access to protected method "' . $name . '" of ' . $this->class, \E_USER_ERROR );

			return $m->invokeArgs( $this->object, $args );
		}

		return $this->trigger_error( 'Called to undefined method "' . $name . '" of ' . $this->class, \E_USER_ERROR );
	}

	/**
	 * get property of wpdb
	 *
	 * @param string $name
	 * @return mixed
	 */
	public function __get( $name ) {

		if ( ! $this->object )
			return;

		$prop = $this->reflector_object->getProperties();
		foreach ( $prop as $p ) {
			if ( $name !== $p->getName() )
				continue;

			if ( $p->isPrivate() )
				return $this->trigger_error( 'Access to private property "' . $name . '" of ' . $this->class, \E_USER_ERROR );

			if ( $p->isProtected() )
				return $this->trigger_error( 'Access to protected property "' . $name . '" of ' . $this->class, \E_USER_ERROR );

			return $p->getValue( $this->object );
		}

		return $this->trigger_error( 'Access to undefined property "' . $name . '" of ' . $this->class, \E_USER_ERROR );
	}

	/**
	 * set property of wpdb
	 *
	 * @param string $name
	 * @param mixed $value
	 * @return void
	 */
	public function __set( $name, $value = NULL ) {

		if ( ! $this->object )
			return;

		$prop = $this->reflector_object->getProperties();
		foreach ( $prop as $p ) {
			if ( $name !== $p->getName() )
				continue;

			if ( $p->isPrivate() )
				return $this->trigger_error( 'Access to private property "' . $name . '" of ' . $this->class, \E_USER_ERROR );

			if ( $p->isProtected() )
				return $this->trigger_error( 'Access to protected property "' . $name . '" of ' . $this->class, \E_USER_ERROR );

			return $p->setValue( $this->object, $value );
		}

		$this->object->{ $name } = $value;
	}

	/**
	 * trigger a native php error
	 *
	 * @param string $msg
	 * @param int $error_type
	 * @return void
	 */
	public function trigger_error( $msg, $error_type ) {

		$backtrace = \debug_backtrace();
		$callee = $backtrace[ 0 ];

		$msg .= ' in <strong>' . $callee[ 'file' ] . '</strong> on line <strong>' . $callee[ 'line' ] . '</strong>';

		\trigger_error( $msg, $error_type );
	}
}

Plugin_Model.php

<?php

/**
 * reflector of Project\Plugin\Data_Model
 */
namespace Project\Theme\Reflector;

class Plugin_Model extends Class_Reflector {

	/**
	 * constructor
	 *
	 * @see Project\Plugin\Data_Model
	 * @param mixed
	 * @return Project\Theme\Reflector\Plugin_Model
	 */
	public function __construct() {
		$args = array( '\Project\Plugin\Data_Model' );
		$args = array_merge( $args, func_get_args() );

		return call_user_func_array( array( $this, 'parent::__construct' ), $args );
	}
}

usage.php

<?php

/**
 * example usage in the theme
 */ 
namespace Project\Theme;

$plugin_model = new Reflector\Plugin_Model( $some, $args );

# if the plugin is active
$data = $plugin_model->get_cool_data(); // gets the expected results

# if the plugin is deactivated
    # if WP_DEBUG
        $data = $plugin_model->get_cool_data(); // NULL, Produces a E_WARNING
    # else
        $data = $plugin_model->get_cool_data(); // NULL, Produces a Notice

Published this, before logged in here: https://gist.github.com/anonymous/6776250