Is there a way to get the composer autoloader instance inside a Symfony2 Controller?
Yes - there is a way.
And assuming that you want to know how to actually get the loader then you can do this in your controller:
class MyController
function myAction()
{
die(get_class($GLOBALS['loader'])); // Composer\Autoload\ClassLoader
Should you do this? Probably not. In most cases you can tweak the loader in the app/autoload.php file.