One of my favorite snacks to eat on daytrips are hardboiled eggs. You could bring chili/spice mix packets with you to add flavor on the road (I like buying some labeled as “hot pot mate” from my local chinese market). Not much of a meal by themselves of course, but can be a very filling supplement to something like a sandwich or a salad. Good as in between snacks too.
I was wondering the exact same thing a few days ago and spent some amount of time googling it.
There’s already such a project actually, called Swanye. There seems to be a single person working on it, so if anyone has time to help I’m sure it will be appreciated