I'm still not sure how likely it is considering the mii costumes were added not that long ago, but man I would really like to see Tails or Knuckles.
Or even better: Eggman
They were probably added before Pass 2 was under consideration, which may have been as early as Hero(since it's the earliest case where some costumes could've been moved to Pass 2. Geno and Chocobo Hat are both missing. We also saw the leftover data slots that pointed to way more characters than Pass 1 quite a bit before Hero was announced, so it was pretty easy to tell another Pass or bonus characters were thought of). So maybe around March or May of last year? Which gives a lot of time to renegotiate how to use the remaining two SE costumes(and possibly not license one so he can be playable). Licensing takes time, of course, but they clearly did have time. At least a month. Since the two were recorded in May, to be ready for June, and they were recorded together.
Unfortunately that also means, if this theory is correct, anything beyond Joker was likely determined with Pass 2 in mind and the costumes there were probably not going to be playable characters either. Not that they can't be, but it looks a lot less likely. So Tails and Knuckles are in a pretty good spot. But those and only the base game costumes appear to be as plausible as any Spirit right now from the base game(and possibly later, depending various contexts of when they were figured out. Some might've been done massively in advance. They take a lot less time than costumes since it's more the specifics and just throwing it all together. You don't need to do model coding. The game already has settings for all kind of spirit battle hazards. So while it obviously takes time, it's probably fairly easy to do as is. There's a lot more spirits than costumes, too). AT's are a different story and opinion. Especially since we don't have a clear answer with Spring Man beyond "he didn't really mention the AT", but we don't know if he was just outright considered at that point by Sakurai, and even then, he was given a character choice by Nintendo first, not picking a list, so who he considers means little for everyone beyond PP among all the DLC. Not that it's impossible for Nintendo to give him a franchise and let him choose who could work the best, but there's nothing to suggest that as plausible either.
It would be nice if we knew more of when the passes were started to be determined. We only have small tidbits at best. And nothing even like that for Pass 2. Pass 1 already told us some information for when Banjo & Kazooie were on the table, for instance. But beyond that, we just know the end date. And of course there's going to be claims by insiders, which is still more helpful than nothing.