Doing this we're able to load our example model into the admin, view it and all related bits and even save without issue. There are some issues when using foreign keys from separate databases in list views in the admin.
Most of this is documented at https://docs.djangoproject.com/en/3.1/topics/db/multi-db/. We've just modified it a bit so things live in settings.
Names for things might seem weird because we've changed them to protect the innocent...