At the risk of coming up with a dumb idea, has the following been considered?
It's apparent that the hopper cars are not coming along as fast as anyone hoped. So in the interim, how about tying a dump truck onto the bed of a flat car, with the rear just about even with the end sill. It could be left on the car and filled with a front end loader. And when dumping, the bed could be slowly raised as the train moved, thereby spreading the ballast.
I don't know the method being used now, so I'm talking with very limited information.