Yes, the park empties out after midnight both days. We were in the parks on 12/30 from around 6p-3a, and then 12/31 from 6:45a to 9:30a and went back around 7p until a bit after 2a. (We napped in between and slept all day the 31st, basically.)
Morning of the 31st, we hit 7DMT at RD, went to Pan, then hit BTMRR when Frontierland opened before an 8:30 BoG breakfast. We had some hiccups in our touring plan so it wasn't spectacular, but still, three headliners done.
Night of the 31st, we arrived around 7:30? 7:45? and rode BTMRR with a FP. We got stuck via the MSEP and then my friend had a small medical issue so it was 8:55 before we queued for Pirates. It went down just before we got to load and down again just before we got to unload (including an announcement that they were going to "power down and immediately power back up") and it was maybe 9:45 before we abandoned ship. We went to 7DMT to use our FPs then were seated at our dessert party table by about 10:20 or so - early, which surprised us.
We hung out until I'd guess 12:20 or so, used the restroom and then walked onto TTA, Buzz, Small World, Little Mermaid, teacups, met Anna & Elsa (5 minute wait - had been the prior night too) and got into line for 7D at 1:55. We walked through the castle at 2:22 as kiss goodnight was finishing. So seven attractions in under two hours.
Now... when we left after breakfast? The crowd was so thick that we glanced left towards the walkway to the CR and decided it was more feasible to make it to the resort monorail than across the throngs of bus/ferry people awaiting bag check. (And I really hate taking the monorail.)
I have photos, if anyone is interested and I can figure out how to post them.