Parties can lobby to have the other person change the feedback, but only if they lobbied party wants to. Seems like a workable system to me and allows each to be honest without consequences like eBay's original system.
E: Also just had another idea. You could have a way to edit your feedback after the original deadline in case of fraud. If you open up a dispute like the one posted then Ebay could review the edit's on a case-by-case basis.