Hi, D.,
You've gotten lots of good suggestions. Because my daughter detests sandwiches, I have to make lots of soup, spaghetti, mac and cheese, even chicken nuggets, in a thermos. If you don't have a thermos, go get one, you'll need it! :)
Though she won't eat lunchmeat on bread, she WILL eat it if I just roll it with a strip of cheese and put a little toothpick in it (she especially loves the fancy ones that people use for appetizers at parties). So maybe your daughter would, too?
As for weight gain, after school PB snacks are a great way to bring up the low blood sugar (and improve moods until dinner) and milkshakes (with some protein powder) are also good for weight gain. (Both my children have less than 3% BMI, so I know what it's like to have to try and get some weight on a child!)
Good luck to you, and kudos to you for accepting what is, rather than being one of those small minded people who are too lazy (or rigid) to think of anything else for their child to eat besides PB. As if that is the only food in the world that can be packed in a lunch! And that lunchtime is the only time the child can eat PB!!! Ugh! The ignorance............