There is much support for placing broodies in a wire cage with cool air moving underneath to help break them "gently". As regarding putting a little weight on her, scratch (corn) can work as can most other "treats". My chooks always go nuts over scratch and dried mealies. Unless you are in dire worry about her weight/condition, focusing on better-nutrition might be a better long-term solution. Electrolytes in the water, high-animal-protein foods like canned tuna, chicken or even scrambled eggs would be my first go-to's.
These would ensure she is getting a proper nutrition boost to more quickly get her back to a state where she can again self-regulate her own weight/condition. An overweight chicken is NEVER a healthy chicken.