Hey, Warren...I had the same concerns about shoulder harnesses slipping off our shoulders continually. We are not large folks, but the straps would not stay put. Ever. Even using a large loop of 2 inch industrial velcro horizontally between the shoulder straps would not help. If there was a sudden stoppage of forward motion of any sort, our faces would strike the dash easily as bodies are quite elastic when severe deceleration occurs.
FD/USA sent me an MRA when I explained what I wanted to do. I understand your geographic dilemma on approvals. I took the F/D harnesses out of the airplane and had a joining strap stitched to the straps at chest level using what are called "cobra clips", very flat locking devices used in utility company fall protection safety harnesses. Designed to be resistant to being opened accidentally and very flat, no comfort issues. The strap has never creeped at all in 6-7 years.
There was a company in Phoenix, Arizona specializing in industrial fall protection that did the work for me and were happy to help. (Cash, no paper to make them comfortable with liability...$50 bucks for both and done!) Just a suggestion if there are any "industrial grade" fall safety harness manufacturers in your area. HOOKER makes a great product, but removing and replacing is way different (paperwork) deal than a small modification to enhance personal safety. I'd wager nobody would ever know/notice the mod. It isn't airframe, power plant, or avionics...so maybe the airplane came that way...?
If your issue is that the harness is coming loose from its attach points on the airframe, I can''t help you there.
By the way, I visited Sunshine Coast some 20 years ago and did some soaring in the Glasshouse Mountains with an airline pilot in a Grob 109 motor glider. We cruised about 30 miles of the most beautiful beach at 300 feet with nary a person in sight anywhere. I love your home!
Blue skies!