Node.js changin the pathname of a http-proxy request

I'm using the RoutingProxy class (found in the http-proxy package) to proxy certain requests in my node app. I'm having difficulty, however, in adding a prefix to the target path. For example, I'm trying to proxy http://localhost:8080/stylesheets/main.css to http://172.30.6.11:51161/mysite/stylesheets/main.css.

Here's a dumb example of what I'm trying to do:

// controllers/proxy.js
var httpProxy = require('http-proxy');

exports.request = function(options){
  var proxy = new httpProxy.RoutingProxy();

  return function(req, res){
    req.url = '/mysite' + req.url;
    proxy.proxyRequest(req, res, options);
  };
};


// app.js
// ...
var controllers = require('./controllers');
app.use(controllers.proxy.request({
  target: {
    host: '172.30.6.11',
    port: 55161
  }
});
// ...

Unfortunately the prefix is never added when calling the target. Does anyone have any idea on how I can make this happen?