Craft 3 – Render template as if it were a site request
I suspect not without manually manipulating the path before the Request class is initialised. 

One possible alternative solution would be to use Guzzle to call the front-end URL rather than the CP URL, which will render the template with getIsSiteRequest set to true, for example:

$client = new \GuzzleHttp\Client();
$response = $client-&gt;request('get', 'http://url/front-end-template');
$body = $response-&gt;getBody();


It would also be possible to create your own controller which fetches the template and to hit it with a front-end action request:

$response = $client-&gt;request('get', 'http://url/actions/plugin/controller/action');