It'd be hard to pin down exactly who, but I feel that Furukawa's the safest bet. He's not only Nintendo's President, but also Ultimate's Excecutive Producer. Outside of him, I could see the producers and general management also being involved in making that decision.
AFAIK Plant wasn't finished on time anyway, so I'm not too sure how much of a factor that was. The compromise was likely Sakurai getting absolute final say in exactly who (as opposed to Corrin). And Furukawa's evidently more than okay with that, the results have been splendid it seems.
Aside from that... just came up with a little forum game:
Suppose Kirby's Inhale got a buff, namely that he gains the opponent's last used B move (Side-B, Up-B, Down-B, Back-B in Terry's Case, or Neutral B) with the same hats. Which move would be the most fun to use as Kirby?