Is it there a way to make duplicity backups to Google Drive?
As far as I understand, WebDav and FTP are not supported by Google Drive. But maybe there is an extension to duplicity in the works?
The only way (IMO) would be to configure a Windows system with Google Drive (there's no Linux client yet, not sure about OSX), share the Google Drive folder, and then mount it on your target *nix system with Samba and use it as a backup target.
It would be convoluted, prone to errors and breakage, and probably perform pretty badly. But you could, if you really had to.
There used to be a plugin for GoogleDocs (announcement), but that one seems to have stopped working at the time GoogleDrive was introduced. I shortly looked into it, and it seems to me that some of the Google GData client libraries (which the plugin uses) still need to be updated.