I'm also curious about the answer to this. An open-source third-party RCS implementation would be a massive boon to the ecosystem. It's obviously theoretically possible for non-Google-Messages SMS apps to support RCS (since vendors like Samsung were able to add it to theirs before deprecating it in favor of Google Messages), but it sounds like that's a non-trivial task (which is probably why said vendors ended up deprecating theirs in favor of Google Messages).