Well blow me down: when I go to the Amazon Drive page there's now an "app" button and if I click on it there's something that implies it's a FS driver. Either it's new or I was blind when I looked at Amazon before.

Prime gives you free Amazon storage, so I had a preference to use it instead of paying for DB. I'll give it a try, thanks.

So I tried it and found it silently failed to upload part of my hierarchy. That was pretty discouraging!