At the moment, you can convert an email to a calendar event and a persistent 'mid' link ('message ID'?) is attached to the event enabling you to go from the event back to the linked message, wherever this may be relocated. This is great. It would be better if we could also manually generate a mid from a message manually and add it to an event, as well as edit, replace, delete the one that may already be attached.
Even better, that mid link would ideally work in other documents or on the desktop and the file manager. Clicking on it outside Thunderbird, would open Thunderbird and locate the message. Great for local referencing in documentation and creating shortcuts.