I don't think I would like being the partner of someone who isn't out. Especially on a special occasion, I would want to be able to be spontaneous and hold his hand or kiss him and all the things you normally do without thinking twice. Having to refrain from doing it would make me feel wronged by him.
This being said, I would probably feel worse not being able to spend said special occasion with my partner at all, and whether he comes out or when isn't my decision... But it kind of reminds me of what some gay people say about never dating closet-cases because you'll come after saving the appearances... It's kind of the same thing. If he's not out, pretending to be mono comes first, and you come second. And that sucks.
But you might be fine with it, since you didn't mention that aspect at all in your post. As for her disliking you, I think you are probably imagining things, because due to the situation being unclear, if she realises you like each other she might perceive you as "the other woman". And nobody wants to be seen that way.
I think if you go you should just relax and be his friend for the night and have fun as such. Either way, have a "real" party in which you can be his girlfriend all you want, because you need to be able to ever so often.