Unaware of other open source projects using RCS. However RCS on Android is currently implemented with code across the OS, Google Messages and Google Play services. To start it would require sandboxed Google Play for RCS activation and so on. It would be less reliant, but not entirely separated.
RCS isn't an open platform at all compared to SMS/MMS. It heavily depends on proprietary Google and carrier infrastructure. We can start by replicating Google's approach in the messaging app they have and then we can work on only using carrier services for carriers where it's actually supported to cut that out.
RCS support is simply for added compatibility and feature pairity. People should use a secure messenger not relying on mobile carriers as a first choice, which our messaging app first run tells you about.
