I know there are no rules, but one can be made.
It's too subjective, now. I'd just make the bat crossing over half of the plate a strike. On the play you mention, it might be unfortunate, but you could allow umpire discretion to over rule the robo ump in just these cases. Many times, a batter trying to get out of the way, moves the bat, but it never actually goes across the plate- it is swung past the plate but not over it.