All of the characters? No.
In 2007, the Brawl demo, in OCTOBER (there's a vid of this roster via Gamespot, I believe) was this:
That's 14 characters out of the 21 or 22-ish that we knew of via the Smash Dojo. It doesn't even have Snake. This was only a few months from the game's release, and well after the E3 in which we originally thought it was coming that December (this demo is after the delay for Sonic).
The demo only had 6 (counting ZSS) newcomers out of the 8 we knew.
Not that they will necessarily stick to what they did last time (I mean, Mario Kart 8 had every character but 2 at the last demo), but to expect all 29 is ridiculous. Especially when they'll likely show off newcomers & vets AT E3 and want people to try them the same day.
If I had to guess, the Demo roster will be mostly the characters from SSB64, a few extra vets (specifically, Pit and the characters who were split from their transformations, maybe Bowser), Villager, Mega Man, Greninja OR Rosalina, Little Mac and any characters they reveal at E3 itself.
But that's just my opinion.