Annie, I did a new years eve party 7 days before I had Colt....
I went to a New Year's Eve party 4 days before I delivered, and felt fine. Even wore high heels and did not feel off balance. (They assorted oddly with my ballonlike face, but not many people were looking at my face, thanks to the low neckline of my dress.) You'll probably do great!
You could go as Octomom, minus the babies...see how many people get it:
http://mcnorman.files.wordpress.com/2009/03/octomom-pic-mardi-gras3.jpg
If I am feeling up to it, my DH and I will go as a priest and a pregnant nun. I've always wanted to do that so this is the year! But, that is less than 3 weeks from my due date, so we'll see.
Green tights, and a little leaf hat with a green viney thing on it (fat green pipecleaners from the craft store work well) and a whole bunch of purple balloons all over your body. A bunch of grapes!
Depending on how religious you are or not, you could go with humor and do the pregnant bride or nun. A belly dancer could be fun too. You could also add equal padding to your back side to match your front and make the whole circle into a pumpkin or a happy face.
T-shirt maybe. Do not want to scare any of the guys with my stretch marks. :)
wear a sports type bra and paint your belly like a jack-o-lantern (orange background with black face cutouts.... or you can do the same thing with a t-shirt, if you don't want your bare belly showing.