In my opinion a centralized authentication platform such as that requires a single point of failure or a level of trust between instances that isn’t and in my opinion shouldn’t be allowed as it would increase the attack surface for bad actors to exploit.
I think the best way would be for the community to create a docker image or other out of box solution that makes it easy for instance hosts to support multiple services on different subdomains from a single endpoint with shared authentication and as such your lemmy.example.com credentials would work for mastodon.example.com and would work for pixelfed.example.com and would work for peertube.example.com and so on and so forth.