I had another thought on this. If you made the magnetic holder round you could make a cup to go over the front of it, bore a hole in the center of the cup and you could use it to center work. As an example, if you had a plate that you wanted to offset bore a hole in, you would just need to locate where you want your hole and drill a pilot hole in the plate. Then you make a pin that fits in the pilot hole that can be screwed to the center hole in the cup, slide the plate onto the pin on the cup and place it on the magnetic holder, allowing you to quickly locate the work and mount it in the chuck.
This is most likely too much work for a single piece but if you make a lot of flat parts it could save you time. Or for people that struggle to set up the four jaw chuck this should place the work almost perfectly centered if you fit the parts for the magnetic holder together so they are true.

Hope that all makes sense.