It's not there already? I noticed that when you click on the Play Games icon on the main screen, it lists all the unlocks. Mind you, once I log in, I don't see my unlocks showing up as unlocked in the Play Games list--but I figured that was just because I didn't discover the sign-in until after I'd already unlocked them.
It apparently works already. If you're signed in and you press the "Unlocks" button, Play Games registers all your previous unlocks....I just discovered that.
Hmm...actually, the achievements show up on Play Games on my other device, once I've signed into STF there...but they don't transfer to the STF unlocks. Rats...with no ability to root the Chromebook now, that leaves me stuck if and when I ever move to another device.