LASM, I hope I'm not too late with this comment... There was club in southern New Jersey that I visited a while back that had the hill for classification. I had never seen one before that, and so of course I asked a bunch of questions about it, and here are a few things that I can remember about it:
1) They had to go back and add the air blow because some cars would simply fly down the hill. To determine which cars needed how much air, they added a letter to the beginning of the car's number. This letter told the operator how much air was needed to keep a decent speed.
2) The layout they had used code 80 rail. The yard inside used code 100. This was part of the design because they were afraid of derailments. When cars would collide (couple) at higher speeds. Apparently this didn't work as well since they still went back and added the air blow.
3) The uncoupling magnet had to be moved up the hill a small amount. During construction and testing, they found that if the coupler was being pulled by the weight of the car looking downhill, the uncoupling rate went down quite a bit.
4) One thing I learned about the hill - it was really fun to watch it in operation. The amount of space they had for the yard alone was about the size of your entire layout, so the sheer size was impressive in itself.
I don't remember if they used an electromagnet or not, but I would think an electromagnet would be better. This way, when pulling cars out of the yard at slow speed there is no concern for cars falling back into the yard. Unless of course you can keep some speed when pulling a train out of the yard.
I'm looking forward to seeing what you come up with! Good luck!