Route to Controller action (not in a plugin)
As it turns out it's not really possible to break out of the intended craft workflow. If you don't want to put additional business logic in plugins, you're only option is changing the index file. Which I did like this: 

try
{
    $app-&gt;run();
}
catch(CHttpException $exception)
{
    // Implement custom route logic here
    // If nothing matches your own routes, just rethrow the $exception
    // like so:
    //throw $exception;
}


I'm more playing around here the learn how craft works, in the end the best solution is probably to just go with a plugin.