How well does your restaurant cater to those with allergies? If you do a good job of it, that can be marketing gold. Take gluten free menus for instance, do you have one? If not it is simple to make one with the items on your menu that are gluten free, so I suggest you do that. Once you have that put together, you can write a blog post (see what I did here), link to it from you main site, an dpost it regularly on twitter and facebook to get the word out.
Unfortunately people with these types of allergies are used to feeling like a pain to the staff of restaurants that are unprepared for them. that makes it even easier to impress them by not only being prepared but also going out of your way to let them know what you have to offer them. That is restaurant marketing, letting them now what you have to offer.
Allergies provide a chance for you to offer more to your guests, to stand out from the crowd, and to provide you restaurant marketing with talking points. Use them and watch how energized your social media followers become, and how you can be a go to place for those who have the allergies you are prepared to handle with ease.