Sorry - but as someone who has organized many an office function, the event is "planned" for people to go and have a good time, because that's the way it is. Just because people have to buy a ticket doesn't mean it is any less planned or doesn't have certain rules of the affair that must be followed - depends on the hall. Like anything else, don't like the rules? Don't go.
At our place of business, we are given the afternoon off to enjoy some convivial moments with our co-workers in a festive atmosphere, which is costing me $30 this year (reasonable for a sit-down meal type party in fair City). Changes are made for vegetarians and people with dietary restrictions only. In this case, the meal is chicken (a rather nice chicken dish, but chicken all the same). Some years there's a choice - not this year. People have worked very hard here putting this party together. I make sure that the large all-office functions get planned by those whose turn it is. I tell them to let me know when people start complaining, so I can - like the mindreader I am - give them a list of the complainers. Same people every year and every function.
I tell these people this: go or don't. It's up to them. And when it's their office's turn to plan the party, I'll tell their boss that they volunteered to be in charge. Usually quiets them right up. If most people are happy, then it's a job well done. Not everyone is going to be happy - not even me.
If the establishment will accommodate the OP and her special request, then fabulous. If not, then the OP can either go or don't. Some places, especially during the holiday season, have more than one group come in and have a rule that food must be pre-ordered in certain amounts, with no special orders on that day. Maybe that's the way it is here - and maybe not. The OP can certainly check. The OP can also check with the organizers to see if her particular dish can be added on to the menu - just so it's ready when the rest of the food is. Who knows - maybe that's the answer.
But I would be very careful in picking nits over an extra-curricular activity - especially one that people have taken the time to plan over and above their regular work. Here, folks can either go to the party on company time, stay at work or take the afternoon off. I view my party ticket as paying $30 to get the afternoon off, plus a nice meal, great conversation and a kickline of dancing reindeer.
I guess what I'm saying is this (and maybe I'm a little more go-with-the-flow about these things): I think that a lot of frustration is being felt over a simple thing. I doubt the organizers spent all day thinking about how they were going to make the OP upset. Go or don't - and this is from someone who has skipped festivities once or twice because she did not care for the arrangements.