thanks for your hints. I looked at the source you pointed to and I am not sure that I am not breaking the db connectivity when editing the class.
To render clear what I am planning. Lets say I have two servers named master and slave. As I understand in the manual, normally I open the Server configuration panel on "master", add "slave" to the known servers and tick the option "Is mirror of" and enter "master" to the textfield. Now, I assume ISPConfig propagates all changes to the dbispconfig database on "master" to the "slave" (or does the slave poll for changes?). For the database connection to "slave", I guess ISPConfig uses the name of the server I entered into the panel (here: slave) and the default mysql port 3306. However, to use stunnel for replication I would need to set the database connection parameters to "localhost" and e.g. port 3999. (stunnel is transparent, so to access the remote database I connect to a local port).
Sorry for bothering and thanks for youe help!