1. I am tired of having to repeat this; there is NO SUCH THING as a "4th Party".
3rd Party refers to any and
all properties that a company (in this case, Nintendo) or any of that company's subsidiaries has no legal ownership of.
2. Despite Nintendo not having legal ownership over Kawashima himself (that would be slavery), they do have ownership over their fictional incarnation of him much like Microsoft has ownership over
theirs.
This can't appear on a non-Nintendo system.
Nor can this.
This can't appear on a non-Xbox related system.
All Nintendo would need to do is get his permission to use his likeness. Which would only be a drop in the bucket. Especially if they could get it to make available hats for Miis on StreetPass for his normal appearance and his Devilish appearance.
Is Kawashima likely? Hell no.
But being "a real person" isn't going to be what prevents him.