They did, but it was blue and clickable, just with "no tables available." This was different, and now I know why, lolol...
Just got off a *very* lengthy phone call with Dining Reservations. Got an awesome CM who was willing to hand-hold a bit -- thankfully -- and together we figured out what on earth the deal is. This all pertains to 12/30 but may also pertain to the 31st.
At first, he pulled up no available reservations for the dessert party on the 30th. Later on, he mentioned that it's because he was looking at 9PM -- the same time as the party on the 27th (which I was modifying to the 30th) -- and there was no party on the 30th. So if you are pulling up reservations on the website and the CM can't find them, be persistent and see if they can find another way to check.
He pulled up a 10PM dessert party, which he was puzzled by as it seemed very late -- since they hold the party an hour before the fireworks. So he searched again and pulled up a 7PM party. We looked at the schedule for the day and they have Wishes at 8:30, and Fantasy in the Sky (NYE) at 11:50. I'm presuming the hold up involved the decision that because there are two sets of fireworks, there will be two dessert parties. Why they are at 7 and 10 instead of 7:30 and 10:50 I don't know, but that's what is indicated. (I will be interested to watch the board to see whether someone gets further info on what the times really are... or maybe if they're moving the times of the fireworks and it hasn't been updated yet?)
And in case there weren't enough surprises -- the price! It's $39.99 plus tax... $42 and change with tax... *higher* than the highest price ($35.99) listed on Disney's site!

I went ahead and moved it, I really hope I don't regret paying the extra cash. (But I know me and I know crowds, and it IS worth the money to be able to see the fireworks without the crush on Main St.)
Anyways -- 30th and 31st are now bookable. Just know which fireworks you want to book for, and know to expect a higher price!