Without considering why a party in a criminal trial might want to do so, are there any obstacles to subpoenaing and examining any of the following people as witnesses during the trial? ETA: Not as eye-witnesses to the alleged crime, but rather as witnesses to the criminal process.
A judge who has acted in some capacity in the case on trial, but who is not presiding. For example: (A) the judge who arraigned the defendant. (B) a judge who recused himself from the case.
A lawyer working in the prosecutor's office that is prosecuting the defendant.
If context is necessary, one reason I can imagine a defense might want to examine these people before the jury is to call into question the motives and methods driving the prosecution of the defendant.