Default image doesn't work with private downloads

When using private downloads, viewing the default image only yields a 404 error page instead of the image. That's because hook_file_download() doesn't feel responsible for that file (it's not in the {files} table, after all) and neither does any other module.

Possible solutions:

• Store the default image in the {files} table, and delete it when the default image or the field is being deleted.
• Do the same thing in a separate {imagefield_default_images} table, and handle that in imagefield_file_download().
• Store the image in a predefined directory (e.g. files/imagefield_default_images) and let imagefield_file_download() access all files in there. (hm... security?)