Can a Google Play app be far behind? It seems pretty unlikely, but never say never.
Fortunately, there's already a way to watch your Google purchases on your Apple TV. You don't need to jailbreak anything, nor even do an end-run around any terms-of-service agreements.
Instead, if I may paraphrase one of my favorite "Simpsons" lines: There's no trick to it; it's just a simple trick. (And there's a hint right in that link.)
First, fire up your Apple TV and load the YouTube app. Select My YouTube, then sign into your account if you haven't already. (This requires a one-time activation, which can be one via phone, tablet or PC.)
Now scroll down until you find the Purchases section. Presto! You just found all the movies and TV shows you ever purchased from Google Play. And you can play them right on your Apple TV, no fuss, no muss.
This trick works on other iOS devices as well, namely iPhones and iPads. Of course, it's not as necessary given that Google Play Movies is already available.