@julian The ICS-Export in my calendar works with an URL like
User ID (here 2) and a special password (here XXXXXX) are given in the URL. Yes, it is possible to use HTTP-Passwords as well, but not all clients support this. So I decided to go for the URL-password.
The script, that generates the ICS must be accessible for public. You simply give each user an extra ICS-key in the database and grab the events directly from there. If the rss-token can be used for this - nice. If not, the addon needs to add an extra column in the db.
phpBB has a migrationsystem, that manages databasemodifications for extensions and I needed a felt couple of months to understand this . If someone gives me a code point, where the special-key-authentification is done and a database access is possible, I can write the ICS composition for export.