can't hurt to try
You may have trouble getting one of the later dinner seatings (to time with the fireworks--those book very early, I'm told.) But, I'd try anyway. We've never gone, and my daughter requested I try, even though it's way past the "180" days in advance. I just booked for the 3rd week of May--an early supper (like 5:25) but that's fine with us, we'll probably head back to the resort after for a relaxing "old fogies" night and leave my "princess" & friend in the park (they're 19.) We actually booked for a night that MK is open late, which I guess usually books up fast too, because people know they'll still have park time after the meal. The Rez CM told me that she always tells people to try-and to be flexible for which meal & which time if they really want to get in. She said even when they fully book at 180 days in advance, they also have cancellations quite often, especially for the castle, because people know if they don't officially cancel the rez. in advance they'll be charged. She said she's sometimes been able to find rez. for even just 24 hours ahead of time. So try it!