How I did it.
I dumped her costume from the Wii U's RAM, and then edited the DLC amiibo table BYAML in the Mush folder to include her character amiibo ID 0x0013. This ensures that it won't break previous saves. I also had to move her to the 'BossNFP' folder, and specify that she would be unlocked through an event course. This ensures that there are two methods (amiibo and the Adventure in Sarasaland course) to unlock her. The costume found in the game files will overwrite the one found in the unknown location.
Her entry is Boss018 (slot 120) so if you name it that in the BossNFP folder, it will not affect any other costumes, and the game, including past Daisy levels locally or online, are fully playable.
I seriously don't understand why Nintendo didn't patch this earlier. It's relatively easy to do...if you know hex editing.