Call get_instance() in Hook Class

To see what's available at what hook points, just open /system/CodeIgniter.php and look for the name of your hook. It will be using _call_hook(name-of-hook);
Like:
$EXT->_call_hook('pre_system');

To know what's available, it would be anything loaded BEFORE that hook is called. You can see that the controller is loaded way after that hook, so it's not available at that point.

The first available hook after a controller is instantiated is "post_controller_constructor"