I cant find anything that would make this illegal...defense clearly doesnt have the same restrictions(9-yard marks, allowing offense to match subs, deception) that the offense does. Unless the coach is on the field or they have more than 11 I dont see anything wrong with this. I know of a few times here in TX since we have the positive points rule that teams that lose on a last second play refuse to play defense for the try since the game is over. I've seen offenses literally walk into the EZ for a 2-pt conversion uncontested.