A friend of mine has a 1999 BMW 740 and his seat belts have acquired a funky odor. Not necessarily a BO smell but kind of a BO and chemical smell together. It is the second 7 series he has had with this problem.
He could try cleaning it with Woolite and water. One part Woolite and 6 parts water. Then try to absorb as much water as possible from the seat belts with a clean MF as he can.