3rd belt in the pre MY2012 rear seat:
The biggest hurdle to just adding another belt to the older back seat is that the that older rear "seat" is actually 2 independent rear seats. The new rear seat is a brand new bench seat that spans the width. Two completely different designs and not a single part in common. So you can't just stick a belt in and say it is the same as the the new seat.
There are two main regulations for seat design and 23.562(b)(1) is the critical case for vertical load. All the seats have some sort of energy absorption mechanism in it. The crew seats and the new bench rear seat use crushable core, while the older rear seats use foam. But the structure underneath the energy modules is just as important as the module itself. Looking more closely, putting someone in the middle they would be riding on the edge of the two seats. Riding over both seats and the seat structure would not pass the vertical test requirements.
The seat belts attachments are also very important. A significant amount of load goes into these safety harnesses and that load has to get transferred into the airframe. To add the the 3rd seat belt we added significant composite and aluminum backup structure to both the floor and the rear bulkhead.
23.562(b) also goes into detail about the specific weight of the anthropomorphic test dummy required for the testing being 170 pounds. 23.785 also details the static structural requirements of the seat and belts. Basically because these regulations are very prescriptive for the specific requirements, you can't just choose to do a different weight and certify the belt and seat for a lighter weight occupant.
So it was a non-starter for us to just add a 3rd belt to the existing seats.
Retrofit new bench seat into old airplane:
We considered the retrofitability at the beginning of the project but it quickly became apparent that the design constraints would not allow us to make this retofittable.
As I mentioned earlier, we had to make massive changes to the airframe to be able to install and certify the new bench seat. New floors, bulkheads, hardpoints, sidewall attach points, aluminum fittings etc. It is not something you can just stick in, or even do a repair to modify it. These are the reasons why you can't retrofit the new seat into the older airplanes.
I certainly don't discourage retrofits, STC, or other modifications. These types of things are done a lot to enhance safety or add utility, and I have flown with several of them, but seat certification and occupant protection is pretty serious business, not straightforward, difficult and expensive to undertake. It is definately not an easy task to certify a seat, or add a 3rd seatbelt in any airplane which is why you just don't see it in Cirrus or any other airplane.
