There is a RELOCATE constant that can be set, which will allow wp-login.php to work, and upon login the siteurl setting will be updated.
We can potentially handle this more gracefully, by not requiring the constant and having some sort of migration step/warning. But do we want that? Seems pretty rare/edge to be honest (possibly the first bug report I've seen on this specifically). And then there's the issue with old URLs breaking, so it's not like we'd want to encourage migrations as we won't be able to handle the old URLs.