hmmmm, you'd also not be really striking that "5th string" nearly as assertively as when you hit it with your thumb, becuase you'd be part-stopping your "chord", like a sort of hiccup, to hit that "5th string" with any kind of separation.

Also, it'd would be fairly un-ergonomic if the bass note of your bum-ditty was the lowest string - as it sometimes is. Every time you hit that high drone string, the return trip you have to make to that low bass note would be the same distance as if you had the 5th string in the conventional place.

It'd be much less effort to just use your fingers and thumb in the conventional way. I suspect you'd have more control using your fingers and thumb even if the banjo was upside down in the way you suggest, becuase you could brush down and then up-pick the "ty" of "bum ditty".