Craft on Heroku with buckateer as a middleman for Amazon S3
I found a solution that lets you overwrites the core file S3.php without touching the core files.

By adding this snippet to config/general.php  

// S3 override  
const BUCKET_NAME = 'YOUR BUCKET NAME';  
require_once('../craft/plugins/gm/S3.php');  

return array(  
   ...  
);  


And duplicate the S3.php file and add it to the plugins folder you can make the needed changes.