A less then perfect shoulder harness will always be better then no shoulder harness.

Adam

Frankly if I was in your shoes I would just talk to your A and P and tell him you want shoulder belts fitted in a way that seems reasonable to him , cash up front, no log book entries, not for attribution and if anybody asks you just say "oh those shoulder belts were on the aircraft when I bought it".

A pilot at my local airport wound up off the end of the runway in an elderly C 172 with no shoulder harnesses. He suffered terrible facial and skull injuries from when his head hit the instrument panel after he jackknifed around the lap belt. If he had shoulder harnesses he probably would have walked away sore but unharmed.

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

There are many STC shoulder harnesses available out there. Our club C140 has Wag Aero harnesses:
Wag-Aero Online Store - August 2012

Also available for Cessnas are the Hooker harnesses:
STC Kits | Aviation | Hooker Harness

Another club I am in just put them in our piper arrow. I was the ringleader that got them installed in the arrow, we used the BAS ones:
B.A.S., Inc. - Aircraft Safety Equipment and Accessories

The Friendly Aviation Association has put out some verbiage which would seem to make it easier to get harnesses put in either as a field approval or a minor alteration, especially for an old airplane like the star of the show:
http://www.faa.gov/aircraft/gen_av/h...lderpolicy.pdf

I don't know if any of this is helpful overseas. I do know that I would rather have a shoulder harness than not, even if I was unsure of the engineering behind it. Anything that takes some of the crash energy and disspates it through your chest and collarbones rather than your head has got to be better.

-- IFMU