Though this procedure is physically valid, there's no way it would have worked in these 2 crashes. It requires a highly coordinated sequence of very unusual tasks between the two pilots, but they were at full task saturation (i.e., brain locked) that they couldn't handle the much more mundane tasks of normal flight. Like, in the Lion Air crash, the CA had the situation stabilized for many minutes, where the plane would trim itself out of trim, and the CA would (electric) trim it back, over and over and over. Very well. But there wasn't the coordination in the handoff to the FO to give him that information ("it keeps trimming down, just trim it back up"). So if they couldn't manage that, there's no way in hell they could coordinate the timing of the roller coaster unload sequence.
Plus, though it is valid, the roller coaster sequence is useless. If anybody has the control authority to pull it into a climb to begin the arc to allow the bunt, they can instead just establish the climb and hold it there for a little while until the speed comes down to where the trim wheel unjams.