Sounds like what the americans call a Jake brake…if theyre fitted to the prop shaft, theyre a great feature, similar to a Retarder on coaches. If they save the operator money on worn brake pads/shoes, then worth the money, but as its an added extra, they may be reluctant to pay the extra to have it fitted…like the Americans who would rather run their engines on cheap fuel 24/7 than buy a night heater !! all in all, it really depends on the price, and i`m certain many operators would be interested.