Something like this: Here is some PHP code and you'd do similar for iOS and Android. Remember to intercept the URL request in WebView! · GitHub
It is good enough to create the account, with the correct info. But it doesn’t override it later on subsequent logins.
According to this I have to enter the Rails app and set things like SiteSetting.discourse_connect_overrides_avatar . I don’t know if this is because it’s old instructions, but the admin control panel doesn’t even contain stuff like “override name” and “override username”.