Annotations are basically binary blobs with a unique name that can be saved and loaded in .kra files. I recently finished a todo from 2005 about implementing them and exposing them to the Python API. You could encode the layer notes in whatever way you want, from plain text to .doc or rtf… and then serialize to a buffer and put the bytearray in a .kra file. It’s the easiest way of extending the .kra format with specific information.