You can substitute a side if it's offered with another entree, e.g. cole slaw at Flame Tree. You can also ask for one of the sides that's offered with the kid's meals, but it will be a child's portion.
If something is only offered farther down the menu under Side Orders or Accompaniments and has a separate price point, such as the fries or onion rings at Flame Tree, they can't be subbed for the baked beans. You can use a Snack Credit or pay OOP for them.