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.