They'd only spoil them as much as an image of the coverart to BbS would, which isn't all that much. I trust that if SE made them, the card would only have their face on it (I was thinking Vanitas' card would have his helmet, not his face) and the ability probably wouldn't be something that spoils BbS.
Yeah, I definitely see that as the way to unlock Days related cards. I mean, to unlock the cards for the characters in Riku's story, you had to first beat his, so I definitely see you having to watch Days before you can find the cards in a special room.