Marth can dair out of it, but not 100% of the time. I think it may involve starting the footstool from lower on their body and not at the top of their head. When it does hit, though, it makes it much easier to tip with the dair for a spike (spacing is tighter on the dair than in Melee now).
Edit: Actually, it doesn't seem readily apparent that where you start the footstool impacts how low/high you'll be before you can do an aerial. I think that's set. What it seems to be, though, is that footstool to dair may be character specific for Marth. Couldn't hit Ganondorf on the ground or in the air in Training. Can reliably hit Shiek on the ground, but whiff in the air. I'm certain that I've gotten the spike off in the air, and others have posted stating they've done it several times in the Marth forum.
Edit2: Also, the direction that you're facing in regards to your opponent matters. If Zelda and Marth are facing each other, the dair whiffs (when Zelda is on the ground, anyways). If they're facing the same direction (Zelda still on the ground) the dair hits. Can't get it to hit in either direction when Zelda is in the air. Versus Toon Link, it hits in all four cases for Marth.
Seems to be a result of the target's footstool "jumped on" animation. On the ground, Zelda ducks her head forward (and she doesn't raise it high enough to get hit for one direction). Interesting property each character has.