Calling controller without a web request

I'm building a RoR service using Facebook's Thrift. Ideally, I would like one of my service methods to be able to call a controller method that does a render_to_string so I can return an HTML snippet from my service. Without a web Request object, is it possible to call a controller method? In understand I could use ERB directly to apply a template, but writing test code for my service would be better if I could use functional tests. Plus, it would be nice to use a more standard mechanism to render my snippets.