I saw in a post on another site...a fellow indicated his "crash book" said not to dye seat belts...apparently weakens the webbing? I also saw it posted that it can be done but only a certain way? I'll see if I can find the info and shoot you a PM.
I think for the liability issue it would be very hard to find someone to say yes it's OK.
I also belong to a cadillac club and a member there was bleaching and soaking his old white seatbelts in all kinds of things for days. He was using bleach, and some other pretty strong things. The general consensus there was that he probably was compromising their integrity.
How will you ever know? Hit the car hard enough to see if they snap? I pretty sure each of us thinks that will never happen to us. That only happens to the other guy! Problem is we are all "the other guy" to someone else.
So what choice do you have? If you contact a seatbelt company they will probably tell you no because they sell and rebuild seatbelts. But how does the webbing get colored in the first place? It's probably dyed before woven. But does that have any affect? Does dyeing after make a difference?
Again, I really don't think you'll find anyone to say yes because of the liabilty. May be a good topic for "How it's Made" or "Mythbusters".