This has been done before. I believe it was in the NL back before there was the DH. Pitcher would throw an inning, then go play LF, the come back in to pitch again. I don't remember all the details, but I know it's been done. It was probably a lefty, who faced a lefty, then played LF for a batter, then came back to face another lefty.
You don't get warm up pitches the second time around (if it's in the same inning). Obviously nobody is going to do that in the World Series and it certainly didn't catch on. I want to say it was the Cardinals or the Dodgers who did it.