I would have replied to the poll, "I used to do it" but that wasn't an option.
My DS has severe food allergies, and when he was really young, we have in fact brought a plain happy meal into a restaurant. I knew he could eat the bun from McDonald's, and most restaurants had nothing of substance that was safe for him. At one place, they requested to take it to the kitchen, and bring it out on a plate for him. That was fine (this place couldn't accommodate his allergies at the time).
I don't do it anymore, as he can now eat a little more variety. I also used to have to bring crackers and things that were safe for him as well.
I wouldn't have ever done this if it weren't due to his allergies. He even used to react to the tables in Italian places (due to the dairy). When he'd touch the table, everywhere he touched would hive up. We even would wipe down tables and the high chairs, but still if there were any trace of dairy he would hive up (he was touch hypersensitive to dairy). That has lessened thank God.
We've been out with my in-laws to restaurants that they chose that had nothing my son could eat, and in a couple of cases all he could have was a bowl of plain rice. Ugh - they chose a seafood place once with shellfish he couldn't have and there was butter everywhere. I was so worried, so he even had to sit in his stroller. Not fun.
One reason I grew to LOVE Disney! They can make him so much great food, and they are so cautious. He never had one problem at Disney. I don't even bring him snacks along anymore (used to).
By the way, my son loves sushi! And it's always a safe choice, with rice and plain fish. I wouldn't bring food into a sushi place.
Oh, MickeyHereWeCome, I have definitely had to bring food to pizza places. My son can't eat pizza. Even at 12, he has never actually been to a "pizza parlour" like Pizza Hut. So maybe the teen was allergic to dairy also. My son wanted a party at Chuck E Cheese when he was 6 (he could tolerate being around it by then), so they made him a hotdog (I spoke with the manager ahead of time to make sure the bun was safe, and it's on their menu) while the other kids had pizza. They made an exception for me to bring in our own cupcakes (dairy and egg free) for the kids instead of their cake. It was more work, but he was fine and had a good time. No one minded that our cake was different.
If we were invited to a pizza place, my son would still have to bring in food. I would call ahead of time to make sure it's okay, and if not, he'd not go.