It is possible by implementing RCS in the app. However, if one's carrier requires USIM authentication, it will require the system privilege.

If your carrier offers no RCS service, then you need to depend on Google. There has been a proof-of-concept 3d party client for Google's carrier-independent RCS service, though.

• quik > https://github.com/octoshrimpy/quik/issues/124
• rcs-fi-client > python > https://github.com/zwyuan/rcs-fi-client
• Replify Messenger > 5ans d'inactivité > https://github.com/vidit135g/Replify-Messenger
• Fossify Message > https://github.com/FossifyOrg/Messages/discussions/34#discussioncomment-11386066
• rust rcs client > https://github.com/Hirohumi/rust-rcs-client
• prevoir accès https://jibe.google.com/avec clée privée
• discussions chez Graphene > https://discuss.grapheneos.org/d/1353-using-rcs-with-google-messages-on-grapheneos/554

Bref c'est une impasse.
A moins de bricoler et téléphones rooté, il reste:

• Google Messages <com.google.android.apps.messaging>
• Samsung Messages <com.samsung.android.messaging>
• Smart Messages <com.messages.chat>
• Beeper <com.beeper.chat>
• Mei <com.mei>

Avec les rapports Exodus très mauvais