I'm testing this more extensively now, but in the five minutes I've fooled around with this I've found that Jigglypuff can do something similar though less safe safe but with more difficulty.
Also Mario is small enough to get past the boundry just barely, but he doesn't have the ability to get up. My initial idea was to have him UpB to wall jump the inside of the thing, but if you come into contact with the inside of the wall, you just appear on the outside like with the inside bottom of the stage.
More to come soon. Keep checking back for results.
Edit: Kirby can also do this, with some interesting results. Kirby's is the most difficult to do of the characters I've found who can do this as there is no handicap for it. Unfortunately Kirby's implementation does not get you the lagless land on the stage, but instead lets you up B through the stage, which in and of itself is still pretty tricky and useful. One nice thing about it is if timed and spaced properly, Kirby only appears above the stage at the apex of the Up B so you appear with a quick right above the ground slash, downward slash, blade wave. I've posted a guide in the jiggs forums on how to perform hers, and I'll probably follow suit now with the Kirby forums.
Edit: Pit is the proud owner of this tech as well, and I'd say he has the easiest implementation and the best use out of it, as his glide does not auto cancel upon passing through the bottom of the stage, so he can easily fly through the bottom of the stage will attacking with an aerial, or waveland for safety.
Edit: So from what I've tested there are a total of 10 characters who can use this to varying degrees of effectiveness, them being, Metaknight, Kirby, Ice Climbers, ROB, Jigglypuff, Charizard, Mewtwo, Pikachu, Ness, and Pit. Other members of the cast probably can, but from my testing those were the ones that I could perform. I've posted guides on different techniques for it in the Jigglypuff, Pit, and Kirby forums, and plan on getting the other ones done in due time.