I would think more of it as a web-of-trust approach rather than something more all encompassing. I realize you have the paid discourse hosting.
I don’t think I’d want a single defined user registry unless I would trust said registry. Like you had it in Revitalizing the Discourse Hub why can’t I be @trajano in my own site.
But if I trusted the other site like stack exchange trusts itself, it could make a good community between two discourse sites. But for the “hub” you can have a prefixed version.
@hub:trajano for instance which will look for the hub version of myself. The prefixes are defined by the site rather than discourse except for hub which would be reserved.
The trust must have the capability of one-way. I.e. hub does not pull from my site, but my site can pull from hub some information (not sure what exactly yet but for sure not e-mails). They cannot query by name either, they can query by hash like gravatar, and it is up to the user on Hub to say “Yah that person on hub is me so please share my info between this site and hub”
Note you don’t actually have to implement hub. Much like git anyone can be a hub, but only discourse.org can be named “hub”
If the trust is two-way then the two sites must agree that their users are the same and there’s no need for user to confirm that they are the same.
Two established sites would likely never trust one another. It’s more of a site branching off to have another instance but want to keep the same community. Like stack overflow.
NOTE seriously though, DO NOT build it with the notion that there’s going to be one central hub. There should be no talk of “Where can I get the API key for global nicknames from discourse.org?” Discourse central hub questions
It’s best to design it decentralized first. Then have the branding of hub for yourself later on.