It would be useful to be able to find out via the API what a user's home wiki is, as in the output of this. @awight says:
"For example: https://meta.wikimedia.org/w/index.php?title=Special%3ACentralAuth&target=Adamw shows that I have gu_home_db = enwiki. There's no API yet, so we would have to code this into Replica, or write the API."