I've gone into MK just before the fireworks begin...and I definitely wasn't alone. We didn't arrive an hour before, really probably just 10-15 minutes before closing. I had been at the Poly having dinner and decided since I had to go to TTC to get my car that I'd just stop at the park and watch fireworks first.
Now...I will say that everything after the train station was jam packed..so we didn't get very far up Main Street, but hey, we were in the park, fireworks were going off and we were happy. Getting out was a bit quicker since we were much closer to the exit though!
If you're talking though about getting to ride something....that might be a bit harder to do "last minute" because so many people have found their place to wait for the fireworks....not saying you couldn't weave/bob/push your way through, but be prepared for some work doing it and some rude comments from those who had been waiting awhile when they think you're trying to find a spot rather than to get through to a ride, lol.