I used to attend the country concerts when they were held at Gillette Stadium. They have the same rules (no backpacks, no food, no beverages, etc.) However, by calling ahead the day before and talking to the head of security, I was able to arrange to bring my supplies in, including syringes, insulin, food and water. You might need to explain to them that you can't stand in lines if your blood sugar is going low and also that being diabetic makes you more susceptible to dehydration. I also found it helpful to mention that they would probably rather have me bringing in my food and water than having to call an ambulance if I passed out as THAT would definitely be disruptive to the event.
If you call, make sure you get the person's name you spoke to, find out if he/she will be there on the day of the event and, if not, what is the name of the person who will be there that day and who security can call if they are questioning you bringing in your stuff. Often times you just have to mention the head security person's name and they won't even bother checking with them, but have the name and a back-up name, just in case.
Ruth