You can't do anonymous mass dissemination with SD cards because of the cost. You can do it with CD-ROMs however. In China, people actually do this and sometimes

You can't do anonymous mass dissemination with SD cards because of the cost. You can do it with CD-ROMs however. In China, people actually do this and sometimes you can find a CDROM with dissident political messages lying on the ground. It's also common to print anti-government messages on the money, which might be a great application of paperback/QR - higher density public messages via circulating currency.