Would it be possible to add a 100% (or an Unlock All Characters) category for the N64 version? I'd be interested in running something like that.

Sure! I'd add that category. It would be pretty tough I think you would need to play through the championship mode a few times to do it. And I also think some of the unlockable characters require you to lose a certain track too. But Definitely! I can add that as a category if you would be running it :)

Doesn't beating Championship mode without losing once unlock all characters or Am I remember things wrong?

Yes beating the championship mode without losing any credits unlocks all the characters, yes! That applies to all the game versions.

No it doesn't apply to the N64 version