OR a call-out from the Co based on a bar altimeter be more suitable
Not really. Bar Alts have an allowable error of +/- 50ft(IIRC), altimeter settings(in the UK anyway) are based on forecast pressure so may not necessarily give 100% accuracy, plus the co pilot would have to know the height of the ground above the altimeter setting datum and subtract that from the reading on the bar alt to know when they were getting near the ground.
A Rad Alt is far more accurate and gives your height above the ground so no need to be doing sums in a time of high stress.