WordPress 3.5: Setting custom “full URL path to files” in the Media Library?

The option name is upload_url_path, and you can still filter it:

add_filter( 'pre_option_upload_url_path', 'wpse_77960_upload_url' );

function wpse_77960_upload_url()
{
    return 'http://subdomain.example.com/files';
}