Never looked in the POH?
I've looked in each of the more than 40 POH/Flight Manuals in my library, before flying those aircraft, and at times to support my statements here.
For example, the most prominent GA plane C172 has it as Part 5 in the POH. Or did I miss something in the discussion?
Section 5 of the 172 POH (and every other modern Flight Manual is "Performance". Section 6 of Flight Manuals, "Weight and Balance" may contain an "Equipment List". This list does not constitute an approved "Minimum Equipment List" (MEL).
The MEL differs from a weight and balance "Equipment List" in that an MEL will allow the pilot to make a determination that the aircraft can be flown with a defect for a limited period of time. Sometimes compensating action or other required serviceability requirements are imposed.
Transport Canada presents MEL information:
(i) Minimum Equipment Lists (MELs) provide an additional measure of control of defects. MELs are lists of systems and equipment installed in the aircraft, annotated to show the degree to which defects may be allowed for a limited period. In some cases, additional operational procedures or restrictions are applied. Procedures to troubleshoot, inspect or secure items prior to takeoff may also be identified as conditions for operation with the equipment inoperative.
(ii) The recent trend in MEL development is that the MEL should become an exhaustive list. As such, any item not listed in an MEL must be operable at the time of dispatch. Items such as entertainment systems or other items installed for the convenience of passengers are usually listed under the heading of passenger convenience equipment. MELs usually address only the operation or non-operation of systems and equipment, and do not cover degraded system performance, such as unusually slow landing gear retraction, excessive fuel consumption, etc. As such, an MEL does not allow for every possible combination of defects, or for the additional workload which may result from multiple defects.
(iii) Application of the MEL does not eliminate the need for the pilot to make his own assessment of the airworthiness of the aircraft, but it does indicate certain circumstances where operation is definitely not permitted.
In the absence of an MEL (which will be distinctly "Approved"), a pilot has very little grounds to fly an aircraft with a defect. One of the few examples which does go through okay would be "a nav light is U/S, but I'm only flying during the day". Otherwise, pilots are not qualified to perform the certification assessment as to the impact of a defect on the operation of an aircraft.
From my certification work, including Flight Manual and MEL development, I can assert that the only way an aircraft could be released to service with a U/S seatbelt would be if that seat was prohibited use, and that
is done on airliners.
If there is a harness, you survive more accidents, yes, but you will also survive much more severe crashes with heavier injuries, thus suffering heavier recovery situations and more likely things which will be unrecoverable. Life by all means and in all circumstances?
I recognize this argument as being the same one to justify not wearing helmets while riding motorcycles. Yes, I suppose it is up to you. If you are an adult, and knowingly choose to not be restrained in the cabin of an aircraft, that is your right (as long as you don't hit someone else when you fly around). I entirely don't agree with that way of thinking, but I respect individual choice. But please, once you have made that choice, and are preventably injured or killed, please don't prevail upon the common health care system. The costs are incredible.
So, to the OP, you're right to not fly an aircraft with a known safety defect. You have no authority to declare it flight worthy with a seatbelt defect. An operator would be wrong to charge you for your use of a U/S aircraft.