I recently went to a catered dinner at my daughter's sorority. I asked the organizer of the event for the name of the caterer and the contact there so I could talk to her about my food intolerances. I called the caterer, who was happy to talk to me about ingredients. As it ended up, all the food was gluten-free except for the salad and they made me a special one with no problem.
I'd just find out who the caterer is and talk to them. Don't do anything that would require additional charges, but at least you can find out if they can accomodate you or whether you need to bring something for yourself. By calling the caterer on your own, you are not making any additional work for the bride/groom ... just tell them you want the caterer's number so you can talk to them about the ingredients since you have food intolerances.
I actually am a wedding caterer with Celiac, and it comes up more than people would think. I usually make ratatouille as an additional dish for those who have Celiac on polenta, it's yummy if anyone wants the recipe. I think that no matter the function, your needs should be met, it would be the same if someone needed Kosher food or was diabetic.