Correct me if I'm wrong, but your concern sounds like that there's no way to tell who is to say you can or can't put a handicap on yourself unless you're the TO and go out of your way to say something like "Handicap fixation is prohibited" or whatever.
In which I agree. I don't see much of a problem with allowing handicaps. Peoples' thing seems to be that "oh, this character's metagame is developed around specific percentages, this is a surgical nerf, blah blah blah!"
... Then how about you make it so that the character is actually better outside these specific scenarios?
I don't know, Meta Knight is still pretty good even if he can't Uair > Uair (times infinity) > Tornado. Just one example, but you know, that's just me. Yay for playing a moderately bad character.