Something to keep in mind is that naming games is not nearly the same as a straight cameo. It's still quite minor in terms of "non-game character content", and shouldn't be taken too seriously.
There's probably fees for Trademark stuff for naming it, if anything, so it's not like it's a big deal.
The Mickey Keychain, for instance, actually is more notable by virtue of being an image of Mickey's head, which also means it would obviously cost more to license. Stuff like Spirits or cameos in a Final Smash is clearly a bigger deal too. A Mii Costume is even bigger than that.
I'm not saying it isn't notable at all, but it's pretty small in the grand scheme of things. I do think they'd easily reference Disney if that's what the agreement comes to. My guess is we'd stop at Spirits since costumes are more than just an image alone. How the model moves is a big deal to the IP holders in many cases, so it'd actually just be more trouble than it's worth. This also applies to any Final Smash cameos.
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As for that leak, if it wasn't known here already, the same user who posted the Oct 5th also posted another possible date. They made a good guess, no more, no less. So ironically them trying to do another leak destroyed their own decent evidence. Several music tracks would've been licensed for with the character a lot earlier than the leak told us, so there's no way it could've been real anyway. Mii Costumes I could believe coming with more than one song, but "several" pretty clearly is implying more than 2-3 at best(which I could legit see Disney agreeing to... but only under some special caveats. Like it costing more than regular costumes, or even a timed offer. You know, something Nintendo wouldn't make the deal of, so...). It's pretty much dead in the water cause they had poor details and pretty much admitted it was false later on by accident.