I would like to throw out there a known issue with incomplete/hacked Characters and modded machines can play them. This can apply with any piece of software.
Programmers/designers are up against a deadline and will want to be as efficient as possible. Really That does not mean that the game will be perfect and that they will finsh everything. That also means that taking incomplete items out of a final product will happen. They will leave code, models, etc that a regular console interface will never access.
Sometimes they are lazy, sometimes they don;t have enough time and sometimes they forget. It happens.
So I think a lot of these "extra characters" that hacked machines can play are just leftovers. MK Staff should put a disclaimer out about these characters.
(And I did not even mention Super-Duper-Flooper-Unlockables.)