Is there evidence that Luhmann's system aimed to create anything from the start in a sort of autopoietic sense? Or is it (more likely) the case that Luhmann saw this sort of "alter ego" emerging over time and described it after-the-fact?

Based on his experiences and note takers and zettelkasten users might expect this outcome now.

Are there examples of prior commonplace book users or note takers seeing or describing this sort of experience in the historical record?

Related to this is the idea that a reader might have a conversation with another author by reading and writing their own notes from a particular text.

The only real difference here is that one's notes and the ability to link them to other ideas or topical headings in a commonplace book or zettelkasten means that the reader/writer has an infinitely growable perfect memory.