At least 95% of people I've played refuse to get off of the ledge at any time outside of the first possible frame; same with the following attack. Also, it's not like it's something that happens to Marth players often. Most attacks could probably be avoided via a fastfall as Marth, but Charizard's ridiculous range on D-Air helps here.
Coming from the ledge to the stage is actually fairly difficult in general in Brawl, so it's not uncommon to see Marth snap to the ledge more than once. If his midair jump won't reach the ledge, you know he has to get to the ledge, so you can dash to it and shield at the edge.
At worst, you'll roll toward him, probably avoiding anything but a well-timed Up-B back to the ledge, dealing ~5%. If he tries to get sneaky and wait before attacking, couldn't you just...hit him? I think this could be a reasonable mix-up.