Q:

Symfony2 / Twig : Generate Alternate Absolute URL Path for use with CDN? 

This stackoverflow solution almost answers my question.  But I want to generate CDN urls in Twig to more than just resources.  I'd like to generate them for dynamic content.  So I don't think an Assetic tweak is the right area to be looking. 
For now, I can set CDN_url in parameters.ini and use that in my urls.  My code would look like this : {{CDN_Url}}{{url('route',{'param1':'value'}} .. Code maintenance is one major reason I don't like this option.  Then you could do things like check the route for /cdn/ to generate the CDN url and everything else to be based on the domain.  I wouldn't have to run around changing a lot of variables.  AND if I wanted to stop the CDN url from generating.  I could just modify the route.  So there's a lot of benefits to figuring out if there's a clean solution to this. 
If, there's not a solution already - how would I start to extend the {{ url() }} functionality so that I could use it like path and url. 

A:

You can do it with the help of the question you linked by following way
{{ asset(path('route',{'param1':'value'})) }}

If you need to handle multiple CDN domains you can do it by following way
In app/config.yml
# app/config.yml
#....
templating:       
  engines: ['twig'] 
  packages:
    cdn1:
      base_urls: ["http://cdn1.domain.com"]
    cdn2:
      base_urls: ["http://cdn2.domain.com"]

And then in your twig template file
{{ asset('path/of/file', 'cdn1')

OR
{{ asset('path/of/file', 'cdn2')

A:

I don't know about the CDN stuff but as far as extending the url function, take a look at:
Symfony\Bridge\Twig\Extension\RoutingExtension
public function __construct(UrlGeneratorInterface $generator)
{
    $this->generator = $generator;
}
public function getFunctions()
{
    return array(
        'url'  => new \Twig_Function_Method($this, 'getUrl'),
        'path' => new \Twig_Function_Method($this, 'getPath'),
    );
}
public function getPath($name, $parameters = array())
{
    return $this->generator->generate($name, $parameters, false);
}

public function getUrl($name, $parameters = array())
{
    return $this->generator->generate($name, $parameters, true);
}

So you could get at getUrl there or you could insert you own UrlGenerator.   Not sure which would be easier in your specific case.   Probably making your own UrlGenerator.