My past experience with the interior dyes was that the parts must be absoutely grease and oil free for the dye to adhere to the substrate. Given that, I doubt that you could get the hand oil and grease off of a seat belt....take a clean, wet rag out to your daily driver and wipe the belts....you will be shocked at what you get.

Sewing a new belt to the metal hardware is not a trivial job....someone must be aware of the loading on the sewn joint...it is not like sewing an interior. There are several companies who rebelt safety harnesses and know how (and have the proper sewn pattern knowledge) to attach the new belting.

mike in tucson