Make image style derivative access based on original image permissions optional

When a user tries to view an image style derivative, image_file_download() determines access based on that users permission to view the original image:

// Check the permissions of the original to grant access to this image.
$headers = module_invoke_all('file_download', $original_uri);
if (!in_array(-1, $headers)) {
  [...grant access...]
}

However, there are some use cases where it is necessary to allow access to image style derivatives, while denying access to the original image (see below for an example).
As this functionality is hard-coded into Drupal (it can't be overridden even by implementing your own hook_file_download()), I therefore propose making it optional whether image style derivative access is based on the original image or not. This could be done simply by checking against a variable that defaults to TRUE.

Example use case:
A photographer sells digital copies of his photos online.
Full-size, original images are uploaded, then image styles are automatically created that scale down and watermark the images for previewing.
All users should be able to view the scaled, watermarked image style derivatives; but the full-size, un-watermarked original images should only be accessible to paying members.