I think it is a good idea, my only concern is will the auger be able to lift the weight of the coal? Most augers run on an angle, they won't lift straight up. If you do it right ,it should be a great way to move the coal.
Axeman-Anderson stoker boilers used an auger fed hopper. The auger was a different design though.
I have been toying with trying to find a 4" or 6" farm auger to use to automaticly empty my ash pan when I'm using the stoker. I could then have a much bigger ash collection box, and have 2-3days of automatic heat for the occasional weekend away.
If you have a link to the 4" auger supplies I'd like to look into it.