maybe pick an ID at random, record it in a "used" list, and store the code in the note as a property? an event handlers(1) could manage removing used codes from the list when a note is deleted. likewise, another handler could check if new notes (ie those imported from another vault) already have an ID and register it as used or ask the user if they want to generate a fresh ID.

[1] https://docs.obsidian.md/Plugins/Events