Actually, I never thought of of using the ol' "Immediately take your base" thing. I guess if I NEEDED out, I could use that!
Otherwise, I think I would eject the kid and have the sub assume his spot at first base.
My question is, how could the plate ump allow the guy to get that many steps away from them. When I have a hit by pitch, I am out in front of the batter immediately to prevent this very thing from happening. I make this "look good" by checking with the batter to make sure he is ok, but again this also serves to put me between the batter and pitcher. Since I started doing this, I have never had a batter do more than just shoot a dirty look to the pitcher.