Passwords are stored in the keychain by default since v1.5. Previous versions stored them in the encrypted Bookmark file for each server - keychain storage was optional.
The 'Macintosh front end' you refer to - is that the KeyChain Access application? If so, your request/suggestion should be sent to Apple since they are the only ones that can make alterations to that software.
At the moment, though, it's not going to be possible to tie together the changing of, for example, an Active Directory password and an FTP server password. They are two entirely different settings and there is no way to 'just know' that they happen to be the same password. I'm not even sure that it would be desirable behavior for the majority of people.
On the other hand, I may have completely misunderstood what you're after...