You can definitely get in early if you have a reservation..excellent idea too!
As far as my personal experience, they were very strict about not letting people in before the start of the party. From 2001-2005 we got in between 3:00-4:00 without reservations and with our party tickets alone. There were party dates in September and October within those years so I don't think the time of month had any bearing, just my observation.
Beginning in 2006 is when they started "tweaking" the policies regarding entry into the party, (there was a whole lot of concern when they started doing this too). They did this because a lot of non-paying guests were just "hanging around" after park close and being allowed to stay in the party without paying (saw this many times early on and it was annoying). They then started closing certain parts of the parks and "herding" non
MNSSHP ticket holders into one area of the park to "clear it out" quicker..didn't work so well. They did a couple different things in 2006 & 2007 which I can't remember exactly, but seemed to go smoothly.
Last year (I think 2007 as well) they had like a line of CM's beginning at the back of the park and "herding" day guests to the entrance. I would walk through the stores on main street and CM's would be shoulder to shoulder checking wristbands for MNSSHP'ers and allowing them through, and guiding all other guests to the entrance. They also checked wristbands ALL NIGHT LONG which I had never seen done in other years.
That being said, we went in October (Halloween night to be specific) and they had much less leniency than years past. Maybe because others went in Sept. which had fewer attendees they let people in early? Maybe because it was Halloween night they were extra strict? I don't know.
However, I would suggest to anyone planning on attending, especially for the first time, do not plan on getting in any earlier than the actual start time. Just my .02