Comment by AAAAaccountAAAA

You don't really need an OS API for RCS like you need for SMS/MMS, as RCS works over IP network. If you want/need to use USIM based authentication, you need a system privilege, so that you can talk to the SIM card.

Someone has written an open source RCS prototype app, but it is only for carrier-provided RCS and I don't know if anyone has even attempted to use it with Google's servers:

https://github.com/Hirohumi/RustyRcs