I've seen it done quite a bit on my son's HS team. WHen he DH'd one game, he hit for one of the infielders. I don't remember if it was the firstbaseman but I have seen the firstbseman DH'd for in a couple of games. I guess it came down who the weakest hitter was that day regardless of the position they played. I'd say that year, about half the games, the DH was batting for a position player.
I think at the HS level, it's a great move because it can maximize a team's strengths and gives more players an opportunity to bring their strengths to the table and get in the game.