Seat belt cleaning seems to be an area where very few companies recommend using their products.
Many "experts" say the seat belts should not be cleaned with any liquid because of weakening the seat belt material.
Sweat stains are not something that I have worked with, but recommended or not, I have used both an all purpose cleaner, (Meg's APC+), and a tar remover to clean up a seat belt that had been caught in the door latch mechanism. Since the vehicle was never wrecked, I really don't know if that damaged the material or not.
Sweat stains are probably mostly salt stains, but I don't know what works good to remove salt. I would probably try some laundry detergent and some hot water if it were my vehicle. If it were someone else's, I think I would pass.
Charles