[development] modifying the drupal final output..

Hi all.. I'm trying to create a module to modify the final content of the request being sent to the client. which is the best way to do that? I've done some test with the hook_exit hook, but can access the content.. and hook_filter only is being executed for the node's content.. The idea for the module is to change the paths for the themeing files (images and css files) to a) local folder of system, b) another path in the web server.. As files (images and so) may not be referenced using drupal code but just with simple html output from any module I just would like to modify the final result after the rendering process.. Thank you all!!