My cousin's husband has a horrible nut allergy and when he was growing up, his mom made him wear a shirt that said "food allergies, please do not feed"
I think your shirt idea is great!
At our walk they have lots of food and beverages and I'll usually grab granola bars and bottles of water or juice for myself, DH grabs a soda. I bring along snacks and juice boxes from home for DS and we don't usually eat lunch afterwards 'cuz my inlaws have an open house for our team -- lots of food, beer.... Ever since someone once mentioned everyone digging in the community food, even though there's antibacterial gel there, it kinda creeps me out.
Liza