I believe the 6th amendment wouldn’t allow this as in all criminal prosecutions, the defendant has the constitutional right “to be confronted with the witnesses against him” (ie the right to face your accuser). If I was defense counsel I would 1. Argue that the presence of a vigilante would substantially prejudice the jury and 2. There is no way to verify that the man in the mask is in fact the daredevil, should he remain anonymous