Just a minute ago I did discover something I did not know yet: it is possible - to my surprise - that it is possible to retrieve a password stored in a User Parameter using a simple ParameterFetcher!
Passwords stored in User Parameters are encrypted (see e.g. this post), but the ParameterFetcher just returns the decrypted password in plain text... :-(
So everybody with access to a workspace with passwords and FME Workbench can just use this fetcher to retrieve all passwords.
OK - maybe I have been a little naive... But this is surely something to take into consideration.