Use any type of detergent on a seatbelt with incredible caution. Washing Power is possibly one of the worst things to use!
You have to remember the belt is made of tightly woven synthetic fabric, and of course its potentially what could save your life one day, the use of chemicals/detergents, particularly washing powders which naturally contain fabric SOFTNERS is not at all good. I would recommend using nothing more than a damp microfibre.
Slightly grubby seatbelts, or you going through the windscreen? Not a risk I would take on mine, or my clients cars.
Regards,
Gaz
www.detailmyride.co.uk