That's not exactly what I was looking for, but it would work! Our door handles are not round, but a lever which the dogs have learned to open on their own. I have to tie the door shut to keep them out of the bedrooms and bathroom during the day, but then you have to untie and retie it all the time. We thought of a hook lock, but if anyone was back there and tried to get out while the hook lock on the other side was fastened, I'll bet there would be some human yelling involved!
I love the idea of something antique and novel that would work, and this might do the trick.