Years ago RIT used to make a fabric rust remover but i am not sure if they still do. It would be found in the same section where RIT dye and other RIT laundry products are.
I have also used Whink rust stain remover. I think that one is found in the automotive section of stores. It works on getting rusty tool stains out of car upholstery and carpeting. I believe it works on any colored fabric, Just test it first!