You should be able to configure a pre_spawn_start hook in your Authenticator that can make arbitrary calls and store the results to auth_state which can be accessed by the spawner.
Starting single-user notebook with our custom ldap docker image - #4 by manics is a bit out of date, but shows the basic principals of a spawner passing user-specific variables to a spawner (in this case a UID) which makes them available to the user as environment variables.