Hello Capn Notarious,
I've trialled a video camera & screen and it doesn't work. You need 3D vision.
Most of the underslinging I have done has been with a crewman in the back, both long line and short line (AS332L). We eventually justified getting a bubble door which made all the difference. We were no more accurate than flying the load blind but we were much quicker. Still prefer to have a crewman in the back as an extra pair of eyes which have better all round vision below the aircraft.
I can well understand why the full time underslingers prefer direct vision rather than mirrors but I have seen some very good mirror pilots.