I emailed them and was told that the part was unavailable. They offered to send a mailing label to me so I could have the whole item replaced. I have faith in Hasbro but I don't expect to get the same toy in return. However, it's okay with me as I can return whatever I receive to Target, and just pick up another Tank Droid when I see it. I do agree that the Hasbro folks are very kind and helpful people.
I would advise against sending it back to Hasbro at all costs. For whatever reason, email correspondence is pretty much guaranteed to come back with the "We will send you a shipping label" response, while calling them via telephone is much more likely to result in them just sending you the part. I've been using the same phone number since the POTF2 days when I had a problem (unfortunately, my address book with that number is at work right now), and I've never had any issues with them using the mailing label thing there.
In 2006, I got a Vader TIE that didn't include the 1-label sticker sheet. I went the email route and got the "Mail it Back" label response. I mailed it back, and two months later, I got a ROTS Obi-Wan JSF (in ROTS packaging, naturally). Luckily I was able to simply return *that* to Target and exchange it for another DV TIE, but the moral of the story is, don't accept the return label thing. It's easier to just take it back to Target, say it was incomplete, and exchange it for the same item, even if you have to wait on another shipment.