In my newspaper the other day, I came across the following sentence:
Had Princess Diana worn a seatbelt, she may have lived.
This usage is very common nowadays, but it strikes me as odd; I would have used might there.
May does not work in this context. I would use it for when we are not sure if the person has lived or not, and not for a past imaginary possibility. It brings the action into the present, when it so clearly is past. There is a sense in which a shade of meaning is being eroded by this usage and it is not a trend I particularly like.