Update (5 days later): It's added! If you're an Arq 4 user, pick "Check for Updates" from Arq's menu to get it. Go to Preferences, Destinations tab, and click the + button. Pick "Google Cloud Storage" for the destination type.
It's really great. I'm using it instead of Glacier for all my personal backups.
This is exactly what I want for Linux (headless), but I haven't been able to find anything like it. The closest thing I've found is attic, which, while amazing, requires a daemon to be running on the remote end (which is what makes checking, deduplication and deltas fast).
Unfortunately duplicity is very inefficient, it needs to reupload the full set every so often, which takes up a lot of bandwidth. I guess attic is my best bet in the foreseeable future.