I'd love to be added to the support list. Skull Kid was one of four characters I remember supporting prior to Brawl (another one was Olimar, so I didn't go empty handed) and someone I've been supporting since. I'm still kicking myself over the fact that I didn't fill in a Smash Ballot entry, but that hopefully doesn't seem to be the be-all and end-all for newcomers. I was going to sign up to the forum and pledge support prior to the last direct, but I thought that would somehow jinx it and he'd be casually revealed as an AT in the same direct ... Might as well sign up now, though!
I've read some posts saying that Skull Kid has to be in in some capacity, either as a playable character, an assist (increasingly unlikely) or a boss. I don't think that's necessarily true: Skull Kid could have been removed from the game altogether. While that sounds like the worst case scenario, I don't think we should panic on December 7th if that turns out to be true. The truth is that Sakurai has limited time and resources to put Ultimate together. Not all characters he considers will make it to the base game, as others will be prioritised. It could be that this is what might happen to Skull Kid - he wasn't considered a priority, so was cut altogether to be stuck in as DLC as a later date. If the box theory turns out to be true, I'm fairly confident that this will happen to Shadow (who could be a Spirits mode boss, but Eggman seems much more likely IMO).
Being MIA certainly isn't a guarantee that a character will be eventually playable. I highly doubt the Brawl assist trophies that didn't make it to 4 were planned as characters. But this is just a theoretical 'don't panic' scenario if Skull Kid is missing altogether. Even if there weren't immediate plans to include him, it'd be a better situation than if he was a boss (which I'm still pretty sceptical about) which could dampen support.