I think I would go with the insurance company that says it doesn't cover injuries incurred while not wearing a seatbelt.
I bet their rates would be pretty good. :)
And on a side note - what if the state limited liability if other people injured in an accident you caused were not wearing their seatbelts?
Sometimes the best motivators are financial. Heck, it works with taxes. Look at all the people that simply MUST own their own home - no matter the price - just to get the tax deduction.
How about doing away with mandatory speed limits and just say that your insurance won't cover your accident if you were going 110 mph?
Or how about doing away with mandatory drivers licenses and having the insurance company not pay if you drive without a license?
These are EXACTLY the same principle you are proposing with seat belts - no difference.
Mandatory licenses, speed limits AND seat belts are all there to protect us. Why ignore one and accept the other two?