Many of the Cessna and I think some Piper have the hard point for the shoulder harness already in place.
In many countries it would be very simple and cheap to just fit the shoulder harness buy drilling the hole/s in the trim to reveal the anchor nut/s.
Unless a STC covers this then you will need a EO for Australia to do the same and that seems to start at $1,000 per aircraft.
For aircraft without the already fitted hard points, I see two simple (but not cheap in Australia) ways to fit a shoulder harness. A bracket fitted to the back of the seat base or to the seat track.
The later the easiest as it can be like a current cargo net restraint fitted to a seat track.
CASA could give general rules that are considered accepted, as they have in the past for heater disablement. But they will not.