Pudnucker, some further thoughts based upon what you've just said.
The certification standard for your Navion will be some old version of FAR-23 "Light Airplanes". For all reasonable purposes the pilot's harness approval requirements in part 23 are identical to those in BCAR Section S - the UK design code for microlight aeroplanes. The norm in the UK microlight world is 4-point harnesses - nobody's a big fan of Cessna-style 3-point harnesses.
Flylight are a microlight manufacturer - they build and sell the kits for the Sky Ranger. That means two things from your perspective - (1) they already carry well designed and made 4-point harnesses, and (2) they have people on tap who are competent to check aircraft designs. Yes, all their experience is microlights, but as I said, the requirements are basically identical anyhow so the only thing you'd be missing is FAA paperwork - they are able to do a competent job.
More broadly, the same would be true of anybody else building microlights in Britain, so it may be worth going and looking at what's fitted to the aeroplanes at your local microlight club, and seeing what seems likely to be retrofittable into your Navion. But yes, I think you're quite right that it's the attachments that will be your biggest headache.
If all else, drop me an email (not a PM) via PPrune- I'm a CAA design signatory for part 23 aeroplanes, and I'm sure could become similarly acceptable to FAA if that was essential. I do occasional consultancy on stuff like this, and am always happy to accept payment in flying hours in interesting aeroplanes
G