I know this is a bit of an old thread, but an over-running coupler (ORC) would certainly solve the problem of sudden stoppage of a brush cutter. It simply fits into the end of the PTO drive shaft (the end nearest the tractor) and over the PTO stub at the rear of the tractor.
These were pretty much required on older tractors, such as a Ford 8N that I had a number of years ago.
These were pretty much required on older tractors, such as a Ford 8N that I had a number of years ago.