I fit numerous ponies (and my colleagues have done the same for 10 years, its what we specialise in) and we see these shaped ponies quite often - they are not easy to fit. As you say, narrow shoulders, no withers (though even withers don't help on the narrower Welsh As!), wide rear ribcages and forward girth grooves. Fitting a saddle does not rely on supposed physical anchors on the horse, it is about getting a good close fit with as much panel contact (grip) as possible and using the right girthing - choice of straps and choice of girth.
As I said, you can't always sit them back quite as far as you'd like, but it IS possible to fit them without the saddle shooting up the neck. I don't know the explanation for the driving harness but I suspect it is the lack of grip - a saddle has a much larger surface area which you can utilitise, with the right tree, panel and flocking techniques, plus the weight of a rider, to keep it roughly in the right place.