The problem with the whole "free DLC" idea, though, is that the developers probably have no way of confirming that you bought the original game unless it came with a Club Nintendo registration code.
EDIT: Hmm...glancing over the list of eligible 3DS games on the Club Nintendo website, it does look like Nano Assault was a Club Nintendo game. Well, if Nintendo was willing to release that information to the developers, I could get behind the whole "reward" aspect for owners of the original retail title (but just as a "nice gesture". They don't owe you anything).