This basically means adding the feature ourselfs and making our own build, not something we can name Android to give low level access to RCS in modem. This is feature we will just add.
Also on the side note, while in theory possible it will be a pain. We can’t just reinstall one package like modemmanger on typical Linux system, we must make a whole new builds for every device and apply patches again on every major Android update. While modem manufacturers do not even share documentation for them, just share a proprietary Android driver that should be in OEM system. So adding such feature means big reverse engeeniring, big effort for automatic patch and build system, all to support in my opinion already outdated and useless protocol which because it is already obsolete with standard XMPP or Matrix based apps.