Hello there, I'm new. (well, first post at least.)
I get the feeling they'd just replace Campbell entirely with someone else, like Meryl or Raiden (from MGS2 not 4, never 4).
They haven't even used Young Campbell since his original appearance (Portable Ops) might just be game history though.
Snakes appearance, I think they'd leave the same, his MGS2 incarnation seems to be one of the most popular and his Brawl artwork is used so much, even still today.
I'm pretty sure Konami used it recently for their biography for Metal Gear Characters to promote soon to be released Ground Zeroes.
And as far as I'm concerned, I'm confident Snake will return and this month could be a potential reveal for him since March 20th is the current release date for Ground Zeroes.
It seems to me that as for Sakurai and Kojima, they'd be ok with Snake returning considering their past history.
I'd speculate that the real issue would be legal stuff licensing, but even still, was that an issue in Brawl?
And why would it be an issue any way?
"Argh! I can't find a stapler to staple these important documents together to finalize Snakes inclusion in Smash Bros 4! DEAL IS OFF!"