Aegir Drush Drupal 6
I see that you have not returned the $items array before exiting hook_menu. I assume it is a typo, but just incase it is not, you need to insert return $items; before the end of the function.

You need to check whether or not Drupal is able to fetch the value returned by drush_get_option(). 

Can you check in your hook_init(), the value of drush_get_option()? 

Something like the following code should work.

function silverpack_config_init(){
  $test_sitename = drush_get_option('site_name');
  dsm($test_sitename);// Install Devel Module to use dsm()
}


Clear the cache, and refresh twice to see if you receive anything back from Drupal. If it is not returning anything, that means either "site_name" is not registered in your database table, or drush is not getting the context properly.