Is Google Messages the only messaging app on android that allows RCS interactions? On the iPhone, iMessages is the only app that has direct access to SMS functionality, but I understand that this isn't the case on android. Is this true?
For most people, Google Messenger app now uses Google's RCS gateway anyway, so the carrier does not need to support RCS.
I do not know of anyone trying to get carrier-based RCS working with a non Google app using a custom rom with a different whitelisted set of apps, but it sounds to like maybe that would be possible
- to be able to access Internet
- to be able to read incoming SMS messages for identification
- to implement a full RCS client stack of your own.
There is an open source RCS client library in existence: https://github.com/Hirohumi/rust-rcs-core
I am not aware of anyone having been managed to connect it to Jibe servers, though. It is completely possible that Google is somehow actively blocking third party implementations, but I don't know for sure.