I think the best thing is to use the same stuff you would feed crickets or roaches.
You can sub whole wheat flour for bran. It's just traditional to use bran- someone in the dawn of breeding mealworms discovered some wild ones in bran or just tried bran for whatever reason and it worked and it's been passed down as the thing to feed mealworms for many decades now. But they don't need it- or even whole wheat flour. You can feed them anything you feed crickets. Just go easy on moisture- just a bit of fruit or vegetables a few times per week is enough for thousands of worms moisture needs. Too much moisture and mites can ruin your colony.