UPDATE (in case this helps anyone else in the future) I have had the feedback successfully removed by eBay.

At first, the rep (of course) encouraged me to ask the buyer for a mutual withdrawal, but I told her that I shouldn't have to negotiate with the buyer over a comment that is clearly in violation of eBay's "Threats and violent language policy", which states:

Hateful, obscene, offensive, profane, racist, sexual, defamatory, or violent language cannot be used in profiles, user IDs, chat rooms, discussion boards, member communication, eBay Claims, eBay Stores, Feedback, listings, product pages, collections, and any other areas of the site

The rep then tried to argue that, because the former part of the comment was factual ("I did not receive my item"), that the entire comment could not be removed. I then argued that surely the buyer is negating their entire comment by including a defamatory statement at the end ("I was ripped off"). I pointed that the buyer could not possibly know what my intentions were, and that by saying that I ripped her off, she was assuming that my intent was to cheat, defraud, rob (I quoted the dictionary definition for good measure), and that that was false.

Then, she put me on hold to "check some things", came back, and told me it can be removed, which it now has been.

So my advice is: if you have someone leave you feedback accusing you of that, or anything similar, arm yourself with the dictionary defintion of the comment (thanks, @countessaalmirena), eBay's policy, and your (positive) actions (apologising/explaining to the buyer and issuing a refund voluntarily, outside of the MBG timeframe), and go into battle.

Thank you again to everyone for your replies.