[fw-mvc] How to manipulate complete response?

How to push the complete response of an http request into a function and
show its result instead?

I thought I have to extend Zend_Controller_Response_Http::__toString()
but it is never executed in my application. I already searched about an
hour to find the place where to put my function call but I can't find
it. I am using Zend_Layout (MVC).

Any clue where to put my function call?

-- Jan