With very few exceptions we take a flight that gets us in about 9:00 AM. We have a room very few times, less than 50% for sure. We never make any specific requests and always check "room ready early" as our top priority. I have asked every time if there was a room available and only 1 time did they offer an alternate room. And that time, turns out we waited on our originally assigned room as it was really a better option. There have even been a couple of times were we didn't get a room until after 4:00, which would be when you'd already be heading to the party.
If having a room for a nap before the party is critical to your enjoying the party then I would hesitate to suggest you take the chance. You could book a room the night before so you know you'll have a room when you arrive. We have done that when faced with your exact scenario. Last August we were doing the Halloween party the day we arrived. I decided that it was too big a risk to take to hope we had a room for us to get in a nap before the party, and there was no way we could do the party without a nap, so I just booked our stay to begin the night before. Yes it meant we paid for a night we didn't stay but we had a room when we arrived, no questions. We got our nap in.
It's always been my understanding that DVC properties have a well less chance of getting in the room early.