A note to John West,
Maybe someone can identify a source of a track pan which in old days was used by hanging under a open pocket to distribute the ballast. They had opening in the pan to put the ballast between the gauge and outside the rail. Some had sliding doors in bottom of pan to regulate the size of the opening to fit the speed and amount you want to drop.
Not recommenced is the practice of putting a tie in front of the wheel to drag dump ballast. If you want to drag ballast up to rail head, build a plow that will be guided on the rail head and having roller to fit against the wheels so the car will not derail. Build with aluminum so it can be lifted in and out easily.
A pan or two hung by chains off top of car is the best way to use EBT hoppers to spread ballast. PRR