If you know of a way to determine with virtually 100% certainty whether the injured was belted, go for it.
Your solution implied 100% or zero coverage depending on whether the seatbelt was worn or not. On = 100% covered. Off = 0%. I can't think of anything where we assign liability and responsibility that way. Maybe that's how they treat non-coverage of suicides during the first n months of a new life insurance policy.
Or, maybe they offer 20% coverage if they're 80% sure it was suicide. Coverage based on the certainty of the determination might not be such a bad idea.