Sure its fun to watch cars brake to avoid your hand, but I have a hard time placing any part of my body out into traffic to stop speeding cars. I'm just saying that if people are really passing within inches of you, then sticking an arm out to stop them is awfully risky. It may work 99 times in a hundred but that's not really good enough. It also means the OP has to keep constant watch over the left shoulder while taking this corner. Is that wise when he says there are patches of gravel in the right wheel path and that cars try to pass on the right when he's in the left wheel path?
You misunderstand, if you are already controlling the lane by your lane position on the bike, adding the outstretched hand helps to increases control of your space. Body language plays a part in riding safely, to some motorists we are nothing but a pole standing still as we are perceived as having a high amount of predictability. Add a human element, something that removes that perception of "we are going to be in this exact spot now till hell freezes over" and you generally get more consideration of your space.
The outstretched hand should never be used at the last minute and I can't find a reference by the op having trouble with cars passing on the right.