I'd like to have it use the same "password" as the whole device as security code when I boot N900 or when it has auto-locked.

So there would be some central database and "server", which would handle passwords for different applications, a password server that is.
Then the password-server would keep encrypted database, which password would be a hash from system security code which the password server application could get from the system when N900 is unlocked. (Maybe someone got it :-)

edit:
When some application would ask for a user+password pair from the password-server, the server would bring a dialog window which would ask something like: "Application Fennec asks a user+password pair for talk.maemo.org. Do you allow or not?"
Then there would "No"-button, and "Yes"-slide-from-left-to-right-thing.
After user has answered either yes/no, the dialog window goes away and application either got or didn't get the user+password-pair.

edit2:
Password safe could be developed futher to have this server which would serve through "standardized" dbus-commands.

Getting the system security code from the system may or may not be a problem. Anyway, Password Safe could optionally use also that system security code hashed as a master password, so one wouldn't need two "master passwords",kinda.

edit3:
I like Password Safe, because the database is compatible with Linux and Windows password applications. I can easily sync the password list just by copying the file between systems.