BPF
I have no doubts that Harnesses fitted by a competent engineer who knows what he is doing will work correctly or he will not fit them.
Having come into flying from car racing I know only too well how important the fixings are, the angles that they are located and a whole myriad of design details which have to be complied with for the belts to work as they are designed too.
The main problem is getting retrofits into an aircraft not designed for belts! Some maybe good candidates for a retrofit others almost impossible.
Then the next problem will be getting approval so yes you are probably correct in getting an engineer to unofficially fit the things.
Whether an airbag could be used for those aircraft where belts are not suitable???
Pace