Since the crotch strap mainly keeps you from submarining forward in a crash, I would think that you could just create a strap, wide enough to catch two rows of rivets on the seat pan, that is bent up behind the stick to attach the crotch strap. You could even get it tied into the bulkhead where the baggage compartment starts for extra strength.
Basically, the seat would be sitting on the entire strap, unnoticed beneath the seat cushion. Using SS or mild steel bent to conform to the seat pan shouldn't be that hard to do.
For a retro fit this would be a lot easier than getting into the bulkhead behind the stick.
Just an idea...