Nintendo of America tweeted that the Nintendo Treehouse stream on the 12th begins airing at 10am PT, and the Smash Brothers section starts at 1pm. It should still be a good watch despite Smash Brothers 3DS will have already released in Japan, however, and there will be other constant streams by players of the game in the meantime.
The current demo released on the Japanese eShop has Mario, Link, Pikachu, Villager and Mega Man available to play, and the regular version of Battlefield or the Final Destination form of it, marked in-game as Omega (Ω).