However, that is approximately what I was going to do until I saw what the dealer had. It is a aprox. 10"X10" metal door that can be slid up as much or as little as you want to control the speed/volume of coal comming out. He built a little chute extention at the bottom of the hatch. He said mount it in the front of the bin just high enough to set the coal skuttle under it. The leaver gives enough force to be able to close the hatch door to stop the flow when the skuttle is full.
I think this is a prety slick way to fill the skuttle at least until the coal level gets too low to flow out the hatch. My bin is 5' high so I can probably get 3/4 of the coal that way, then slide the whole front door up and out as you suggest with the slats access the remaining coal.
If I don't find a sliding hatch soon, I will build it the way you suggest.