B word "what rot"
So Mr B word I am guessing that you are are a licensed engineer who has extensive experience of composite aircraft maintenance and repair especially Grob 115 and Cirrus types ?
The BRS retrofit for the Cessna is particularly easy as the main structural members that the Kevlar strops attach to are on top of the aircraft ( main spar webs & rear spar frame ) can be reached by one central channel along the top of the cabin.
To retrofit a low wing aircraft the strops would have to be routed along the outside of the fuselage to the engine frame mount structure aft of the firewall and to a hard point in the aft fuselage. For C of G equipment reasons and to attach to the rear spar carry through structure the parachute/rocket pack would have to be mounted in the rear baggage area of the Grob. This means that the strops to the forward hard points would have to go under the sliding canopy and the canopy and the canopy rails would have to be jettisoned during deployment, this would require some sort of explosive canopy ( and mounting rail ) jettison system that would sequence ahead of the parachute rocket deployment.
Unlike the Cessna or Cirrus that have the crew & pax in an enclosed cabin the need to jettison the canopy the crew of a Grob would be very close to the canopy as it is explosively jettisoned and then be exposed to the rocket motor as it fires.
As Mr B word you have clearly considered these issues I would be very interested to know how your engineering expertees would resolve these problems with the Grob and what your estimated cost would be ?