Although it wouldn’t be a ToS violation, it could be a DMCA violation (or a violation another copyright law from another country) to circumvent a method that is used to protect the copyright of an image or control access to the original image.
I don’t think that referer header forgery counts as being “a good web citizen”, and if the DMCA indeed applies, it would even be illegal to distribute such software.
Many import scripts have their own code for attachment downloads, have you considered creating a regexp and applying that logic to inline Photobucket images as well - including the referer?