I did my rescue course in a BP/W as was my "victim". Removing it was no problem. Just pass the first hand and arm under the shoulder strap at belly height. My instructor was surprised at how easy it was. He had an extra BCD sitting in his truck in case we couldn't manage it with the backplates.
- Some courses (rescue diver) require removing casualty's harness on the surface as one of the exercises. It's a completely pointless skill because in a real world scenario, you would just cut the harness. Removing the harness requires some skill and few hints from a good instructor - e.g. deflating the wing and drysuit a bit. If you take the course in a tropical destination, you might need to loosen the harness to cheat as local instructors might not have a clue how to help you.
The key is that backplate shoulder straps are not meant to be tight. The usual guidance is to leave a fist worth of clearance between the strap and your body. The waist and crotch straps keep everything on place.
Edit: this was in warm water with 3mm wetsuits.