For retrofit, I think it is more common to cut 1-2 inch openings through the ceiling or wall face (i.e. cut through plaster or remove pieces of paneling), blow the insulation, and finally patch those holes. And, this pattern will repeat every 5-10 feet, since the bays should be physically divided along their length by fire-blocking if built to code. Each blocked section will have to be filled as a separate step.