I don't see what's so hard to imagine it for the 3DS. I'm sure we will see it next update, and I think it will only kick in if there's a need for a second page.
It's hard for me to imagine how it works because you select a stage by simply clicking the stage's button.
Thus, a button to go to a second page wouldn't come off as an issue.
When you select a character, you place a token over the character's button.
While the issue that comes immediate to mind in regards to a button that switches the characters present on the screen on a game with that kind of selection mechanism is nullified by the fact it's for a particular game where only
one player interacts with the screen, it still raises some questions.
Where do the tokens go when you switch pages?
If the tokens stay on the page they're on, how do you get them on the other page?
If the tokens stay in place regardless of the page, how would that effect selection? (i.e. Let's say you pick Ike. Ike happens to be in a place where Dixie Kong's icon is on the next page. If you switch pages, wouldn't that change your selection to from Ike to Dixie if the token transfers over?)
etc.