Interesting.....George sensei, I've just finished watching your "Irimi" instructional video and I think what you have to say speaks clearly to the flinch response..... "being inside" with your mind before the combative distance is closed resulting in nage's action simply being a physical manifestation of already being mentally inside. This may not speak to the physical symptons of our fight or flight response but, I find, gives me a starting point to work from. Perhaps you could elaborate on that idea if you agree with my statement.
I think the whole idea of "irimi", entering first with your mind and then with your body, pretty much precludes a flinch response. It's not hard not to get the flinch / startle response when you already know you are in an encounter.
On the other hand, it is very difficult to not have the startle response when you are surprised (which my videos didn't really address). This is why predators prefer an ambush.
I think Peyton Quinn's Adrenaline Stress Conditioning work is the most realistic training for the unexpected assault you can find. He's at Rocky Mountain Combat Applications.