I can't answer about the Hingebacks (since I have none) - but as far as the Eastern Box Turtles - a little corn on the cob now and then won't hurt them. Mine love it!
When I had box turtles I fed them corn on the cob every once and awhile, they love it. I also give it to my redfoots and radiated,,,,,, I dont give it to these guys very much at all. Every so often is not going to hurt anything
All my kids get corn on the cob when the prices a right. Or canned corn, i drain the fluids into a bowl of Mazuri and place it all on a food carrier once plumped...