Nearly no GA aircraft are provided with an approved Minimum Equipment List (MEL). Therefore it is very difficult for a pilot to assert that the aircraft is airworthy with ANY defect. At least with an MEL, the pilot can point to it, and say I can fly for x hours with this defect under y condition. As the pilot, you really don't want to be explaining how a person became injured for lack of an airworthy seatbelt - that would mean that you were flying an unairworthy aircraft.
As for any form of restraint, I would not fly the aircraft until it is entirely serviceable, it is your last line of personal defense from injury.