I went with a helmet mirror. In the shop I simulated where a mirror would be above the bar, and there'd be no way I could see around my shoulder. With the helmet mirror I can easily see the stoker's face. Not the pedals though, that's a look under the arm job. We've got power straps on the pedals so his feet are usually secure anyway.
Bar end mirrors would have been looking under the arm, so may have worked fine. The elderly shop assistant (been riding for about 50 yrs) had no idea on mirrors though.