That is a helpful tip, because I hadn’t discovered the ability to grab the message link from a contextual menu.
It gets me close - but not quite there. Of course, this will work with some modifications to the way I do things.
What I can do at the moment is simply have an email highlighted/in the viewport of Mail.app. I can then go to any other application and by typing
elink Textexpander will trigger and replace that with the message ID URL. I don’t have to manually copy the link first.
This makes me wonder if I can configure Keyboard Maestro to somehow interact with Postbox to copy its message ID URL and perform the text replacement instead of TextExpander.
Experimentation may be required.