How to invoke a controller action from another controller in-memory?

I have a rails server that serves mobile clients(native) and a browser
based client. Most of the trafic comes from the browser based client.
The browser based client goes through an intermediary rails server to
get the views. The central server exposed REST based services.

[ Iphone

| |
Android ]<-------------------------------------->| Central
Server |

| |
[ Browser ]<------->[intermediary server]<--------

| |


System is in production for last 8 months. Recently, during one of the
performance tuning exercises, we noticed that XML serialization/de-
serialization costs are very high for ActiveResource communications.
So we are trying to co-locate the intermediary server and the central
rails server controllers in
rails server instance.

I can rewrite the Intermediary controllers to directly use the models.
This approach requires rewriting lot of code and I will loose all the
authorization checks enforced through
the central controllers.

I am trying to figure out how to invoke the central server controllers
from the intermediary server controller without incurring the
serialization/deserialization costs.
Ideally, I would like the existing central controllers to return an in
memory object rather than a XML string.


Central controller:

class ActiveRecord::Base
  def to_ar

class UserController
  def show
    respond_to do |format|
      format.xml  { render :xml  => @user }
      format.json { render :json => @user }   { render :ar   => @user }

Central controller:

app.get '/users/'
@user = app.response.body

Any pointers/advises will be highly appreciated.

PS: I am on Rails 2.3.9.