There is a way to put harnesses in as field approval (maybe even a minor mod) and it's been done on a lot of airframes. I was going to do it on the 520, but sold it before got around to it. Give Morris a call, he'll know: 321 403-8813
Had a nice conversation with Mr. Kernick today. It feels a bit like talking with a legend, honestly!
He was great to speak with and I feel better about the ability to install harnesses. Specifically, he pointed out that installation can be done under Advisory Circular guidance and no STC or 337 is required......that the FAA allows safety equipment to be installed without a lot of red tape (paraphrasing) and that the greater good is served by allowing such modifications.
He has a process to install inertia reels in Commnader seats, depending upon the type and for the rear bench seats, he had a couple of solutions, as well.
On the club seating topic, he stated that Commander seats can be installed in that configuration, with the caveat that they must have headrests. However, he noted many blow off that requirement in the Commander fleet.
Seems like a wonderful guy and obviously, as noted elsewhere, a font of knowledge & history on these aircraft.