We did Boo Bash on 10/5, which was a sold out event. We were prepared for the worst, but it turned out to be fantastic! By 9:30pm, we were seeing very low waits for pretty much everything save for 7DMT. My kids are older and just really wanted to focus on the rides. We saw nearly empty queues at rides like Space Mountain, BTMR, and PoC for the rest of the night. Haunted Mansion stayed a little busy, but we rode it 3 times before we left, and our longest wait was 10 minutes. As I mentioned, 7DMT stayed around 40-50 minutes even towards the end of the party.
Here is a pic my daughter took of us all alone on BTMR around 10:30pm. We were the only ones on the train, and there was nobody left in the queues. When we came back into the station, there were a handful of people in the queue stalls, so we had to get out, but we rode it again three more times as fast as we could get back to the gates.
We headed over to HM and walked through the queue straight to the stretching room, which had about 15 people in it. Then we moved to Tomorrowland and found Space Mountain with an empty outdoor queue:
And an empty indoor queue! There were maybe 20 people at the top in the main room. We rode three more times as quickly as we could get through the queue.
On the last ride, there was nobody in the station queues.
For a sold out event, we were pretty surprised how quickly the park went from feeling busy to feeling very not busy. I would love to hear from someone who was at the same party, and if your experience mirrored ours.