If well trained folks are checking out passengers I wouldn’t be as concerned. But no way could there possibly be that many micro expression experts willing to man security checkpoints. I’ve considered just wearing a t-shirt that says “Just anxiety... I am not going to blow you up or steal your stuff. Leave me alone.”
I don’t specifically know how difficult it would be to train people to spot the responses, but I would guess it really wouldn’t be that difficult. It certainly would, however, require a higher standard for TSA personnel, and this is likely the biggest reason why it will never be done.