Say you are user@server.social and I want to link to the account ExternalUser@RemoteServer.social, but I want to open the profile from *your* server.
With Mastodon, I can do server.social/@ExternalUser@RemoteServer.social, if your server already knows about the remote account, I think? Either way, this is very Mastodon-centric, I believe.
I do know that I can get the "subscribe" URL (authorize_interaction for Mastodon and some other platforms) if I know your username.
So let's say I have an input field on a page that asks you just that.
I can then fetch:
https://server.social/.well-known/webfinger?resource=acct:user@server.social
Which I believe works across all fediverse platforms?