Sorry, but I've been thinking again.
If a defender knees an attacker in the back after he scores a try, following the conversion attempt a PK is awarded "where play would restart" ie in the middle. So far, so good.
Now, what would happen if after scoring a try (ie the ball is dead) the try scorer offends?
The try has been scored and the ball is dead, so I don't see how the try (and conversion) could be disallowed, so would you restart with a PK to the defending side at the middle?