Anamorphism is an interesting way to hide data. It's basically using the relationship between the viewer and the ciphertext as a decryption key.

You can store data in relationships that is impossible to perceive in bitstreams. For example, if you express note IDs as 3D coordinates, then any event that references another event via e tag creates a 3D vector, but the vector is not explicitly written anywhere. It is stored in the relationship and interpretation of the events.