They approve and post dozens of Toy Boxes every week that have nothing to do with the Challenges. There's at least one a week with Hogwarts in the title.
As for being featured, they've only done that once. For Thanksgiving, they picked 5 Toy Boxes that they thought were cool but didn't conform to any Challenge. It would be nice if they did that more often, but alas they haven't.
Still, even without being featured by DI, you can still get a good amount of downloads from the community.