There are some clear caveats. You’ll need a current Android Messages beta (5.2 as of this writing), an app that can see Android activities (such as Activity Launcher) and WiFi turned off. And that’s assuming the feature stays on. It’s possible that Google can shut off access to this solution with little notice.
Despite those limitations, people have had success using this method on a variety of devices, all four major US carriers and at least some international networks. It’s technically feasible to make RCS widely available, then. With that said, enabling universal RCS access may not be just a matter of flipping a virtual switch. Google’s official approach involves users’ clients pinging each other to see if the next-gen messaging is available. It may not want roll out the ability beyond France and the UK until it knows it can easily and reliably push the feature to everyone.