The FastFormPlugin which is in charge of recognising `login_url`, `logout_handler` etc does so based on the urls on the configuration. They can't be dynamic as once the plugin is configured they can't change anymore.
Another simpler alternative might be to just keep the same underlying configuration for all companies but set a value in the session that remembers for which company the user is trying to authenticate every time a company website is accessed. At that point the `/login`, `/post_login` and so on urls could just redirect you to their company dedicated counterparts based on the value stored in the session.