It sounds like this is a party you can't avoid--but if that's not true, I would just find an excuse not to go.
If you feel you should go, then I disagree with the idea of bringing your own food--unless you want to go down in the history of the group as being "the weird one who brought her own food, and what's with that, anyway?"
Go, have one slice of whatever is the least offensive pizza, take a piece of cake and eat as much as you can stand of it, and leave the rest; if there's a salad, have a little bit of that, and make the focus of the event the PEOPLE there, not the food choices.
You'll be hungry afterward--plan to eat some real food when you get home. Or, as someone else suggested, eat beforehand so you're not very hungry to begin with. You do not need to tell anyone at the party about this!
I have noticed the following things since I've been on this path. (1) Garbage cans at parties often have a lot of half-eaten pieces of cake. (2) Also, a lot of half-eaten pizza. (3) Some people at events eat a little bit, and just don't get around to finishing because it's not about the food!
And no one really questions them as long as they don't make a big deal out of what they are doing.
Don't stress over it. Consider it practice for real life, 'cause that's what it is.
Jay