I think rotary dump is the only feasable solution if you have many cars to do. Even adding time for vibrating or picking out, likely still faster than any clamshells. You could also rotary dump at one station, then at the second station, have a dock mounted long crane with just a light scraper on the end. Basically a pick ax on long knuckle booms. Car dumps at station 1, then train indexes ahead. Next car is now dumping at station 1 while 'empty' car is rotated again at station 2 and scraped out with the boom or crane.
I think any modifications to the gondolas makes it immediately a dedicated train, less flexability to add capacity, lots of maintenance, and susceptability to damage to the various mechanisms. There are also major AAR issues in the modifications of old cars. I assume you would want old, cheap, gondolas, not building new from scratch.
coal and grain unit trains typically rotate the car to dump as noted above. With rotary coupler on one end of each car, the trains never uncouple for months or years. Fil on the go, dump on rotary dumper, and back to the mines. 'Lather, rinse, & repeat'.