http://en.wikipedia.org/wiki/Mini-DIN_connector
Even the 128k Mac had mostly standards-based connectors.
Mouse port, 1 eight-bit keyboard bus, 300 baud, RJ11 connector for the Macintosh Keyboard. Two RS-232/RS-422 serial ports, 230.4K baud maximum, DB-9 connector, and sound port for external audio amplifier or headphones.
By the way it's a DE-9 connector (a DB-9 would be huge with lots of empty space). I know many people say "DB-9" when they mean the smaller connector, but then again many people say "narrowband"as the opposite of "broadband" when they really mean "baseband" -- just trying to make up words to sound sophisticated. Yes, even Apple marketing...
In my little social circle - any discussion that becomes overly detail oriented (focusing on details over the core argument) has been dubbed a 'DE-9' discussion.
That's what API design is so important, and so difficult. By analogy the DIN connector is simply calling convention and ADB the actual function calls.
Now, if you just wanted to hack peripheral hardware at the time, you would have invested in a lot cheaper (expendable) computer with a standard serial / parallel port, so I don't really understand what you are saying.