Drag chute. I may be wrong but if the throttle closure as a means of simulating failure was potentially dangerous, surely the manufacturer would be legally bound to advise of this in the POH? I know that the heading Caution or the heading Warning is placed in Boeing manuals if a certain action can cause loss of life etc. Do these headings appear in the POH for the aircraft you refer to?
My understanding (although I know little of these bigger GA engines) is that immediately after the engine failure is simulated by cutting the mixture on take off, within a second of applying corrective rudder the pilot closes the throttle to confirm that the correct engine has been identified. In that case the so called cushioning effect is immediately thrown out of the window because it only occurs if the throttle is wide open (as you said?)
I would doubt therefore that any regulatory authority would certify that type of engine if it could not withstand the closure of throttle from high power. It would automatically mean that any rapid throttle closure in an abort could maybe damage the engine? I thought these engines were reliable??