This is a really good idea in principle, but I'm not sure how it would work in practise.
I suppose there could be a pop-up that appeared as soon as a buyer hit the "negative FB" choice, or better still, a link to open a message screen directly to the buyer. Of course, that would mean eBay would probably have to completely re-programme its site.
The trouble I can see is that some buyers don't leave FB (negative or positive) until late in the process, so allowing the seller a few days to respond could be an issue. What if that conflicted with the timeframe given to buyers to make a legitimate claim?
Judging by policy changes over the last year or so, EBay seems to have been moving away from encouraging direct buyer-to-seller communication, and more toward a system of automated intervention on behalf of the parties, where eBay (or Paypal) is the referee. They got everybody used to the idea, through a claims system, and now "Managed Returns" and "Money Back Guarantee" are more recent examples.
The sort of change to the FB system that you suggest would be a complete reversal of eBay's stance. I doubt we're ever going to see it, but that doesn't mean I don't wish it could be done. Still, I'd wager that the whole FB system is about to be tossed out and replaced with something completely different, and hopefully not worse -- just wait for it in the Spring Seller Update. 
