The harness wouldn't require an STC as it is inside the aircraft and requires no tools to remove/install,
These are not the only criteria for possibly requiring an STC. Restraint means for occupants are a "major mod", and also must comply with TSO's. Restrain of items of mass in the cabin require approval. There would be responsibility for a pilot who allowed the use of "alternate" restraint means during their flight. I would be sure that approvals are correct for the aircraft and operation.