mrXmr I don't know anything about hosting on Tor, but there shouldn't be anything different from other PHP-based applications.
If you can't hard-code the domain name in config.php, you can update that file to extract the hostname from the $SERVER global PHP variables and dynamically return its value in the config array. Everything else should work fine as long as the hostname used to access the website matches with the one returned by config.php during the same request.
Flarum has a noscript mode but it's just readonly and extremely basic. I'm not sure how things work with javascript on the Tor network. Javascript and cookies must be enabled in order to use any of the user account features.
Also of note, there are a few features and extensions making use of external requests so those could leak a server IP. Most importantly the mail sending, but some extensions like social login might retrieve avatars from other servers. Though I suppose social logins wouldn't work or make sense over Tor anyway.
