Hiya you probley could make an adaption by useing a bucking strap.
As if it had a buckle on top it would stop the back band slipping down the loin strap.
It would compensate were the crupper was and the sercoid would not be agrevated and you will still have your full breeching in place if you drove a single horse pony mule or donkey.
Its a matter of thinking of the back strap were the loin strap goes through and also the bucking strap from shaft to shaft through the back band were the loin strap passes through.
What might be a good idear to would be if you had the bucking strap like discribed but with a pair of buckles one each side and a small strap to go over the top or just to make things easer have one end stiched and a buckle on the other side then it would be much easer to pass through the back strap with the loin strap.
That way it will have some freeplay to move and stay in shape as kept by the crupper.
If so a back strap will have to be made also to acomadate the loin strap and the bucking strap.
Please let us know if this has been some help many thanks michael.