Sorry, Linux. I use EncFS already over Dropbox (as pretty much the only way to use Dropbox), but I don't want to EncFS my entire drive just for backups, it seems like something that should be handled on the tool itself. I'm thinking of writing a thin layer over Duplicity's backends that will EncFS-style encrypt files and store them on the various services.

IIRC, EncFS has this option where you can use it backwards. That is you mount a normal file system say /foo to /bar and then /bar contains the EncFS encrypted files. This might be an option.

This is pretty much exactly what I needed. You win this QA, thank you!