I think Chrom kicks most rules regarding promotions out. You have a guy who's a character in another fighters final smash as well as victory animations get his own fighter.I doubt we'll get an AT promoted in the same game(Spirits are significantly more likely), but I can't write off possibilities that can actually happen.
There's no rule or statement by Sakurai, so just constantly saying Sakurai said so in more than one way is pointless. No, it doesn't work that way. You can doubt it, but there's no legitimate evidence that proves it's impossible. It's a waste of time to dismiss it entirely. It cannot be backed up. Being skeptical makes sense. Taking pointless extremes doesn't make for an actual valid argument.
Consider the fact we've had DLC once so far. Smash 4. That means there's no way to use that as a pattern to what could happen. Remember that Ultimate has a completely different design being followed. Waluigi could happen(I can be doubtful, but I see no evidence of it never happening in this game), though it'd obviously be post-pass if ever. Sakurai does change his mind given enough time. The only thing that matters is if the fan demand helps him see a workable moveset. He is not going to conform to pointless fanrules for the sake of it. His only real rule is, get this, no non-game 3rd party characters(due to licensing).
Also, Banjo-Kazooie was completely relevant with Rare Replay being a huge advertised thing(and has been for years on end). He never lacked relevancy during the time he was added to Smash. K. Rool definitely did, though. Last thing at most was a Smash 4 costume. But hasn't been pushed at all otherwise. He was underused too, only showing up in Mario Sports spin-offs. Sad, too, but different team behind the new DKC seems to be the case, but I forget why they aren't using him. At least Microsoft is outright using one of the old (used to be)Nintendo Mascots.
The question now isn't "can they promote ATs/Spirits/Mii costumes?"
It's "Will they promote Ats/Spirits/Mii costumes:.