So, a SSB special demo is out now in NA and EU, for platinum Nintendo club members who are subscribed to the newsletter. It appears said lucky people also got extra codes, and one spending some time looking out may well find a code.
Which I did.
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It seems online play is disabled, and that 5 characters are available : Mario, Link, Pikachu, Villager and Megaman, with only Battlefield as a selectable stage.
Most other functionalities are disabled, including match config (no stock play, boo). You get coins, but no idea what these are for (I heard they transfer to the game, neat).