Not yet possible. Google hasn't provided an API to access RCS from 3rd party apps. They did to Samsung's messenger (1st party as far as their own phones), but as far as I know that is the only other one currently. (Verizon for a while had their own RCS implementation and infrastructure and their own messenger app could access it)

I had this same question since seeing the post about the Fossify phone app. For phone, calendar, contacts, things like those, i dont see what value the official google apps have (other than syncing to your account, but i can manage that myself). For Messages from Google, tho, there is something they provide in the RCS, if only because they block others from implementing.

So, using Fossify Message, for example, sacrifices something of some actual value here...

https://github.com/deku-messaging/Deku-SMS-Android

Supports emoji & images Supports e2ee if the other party also have deny SMS installed Supports message forwording

I've been using Beeper. It's open source and built on Matrix bridges, also open source. For RCS you have to link it to your Google Messages