Would it be possible to implement RCS in Signal, if Google/Apple allow access to their RCS cloud infrastructure? Has anyone heard of speculative plans for that?
I'm sure Apple will block 3rd party apps from accessing the device's RCS messaging APIs just like they do SMS/MMS/iMessage. Google unfortunately does the same with RCS (unlike SMS/MMS) in Android, so it's not really feasible for 3rd party apps to integrate RCS support anywhere even if they want to.
Actually Google Messages extends beyond the universal profile. If you check the current spec, there is no encryption nor reactions, yet Google messages has them. Apple Messages won’t for RCS.