Here’s is an interesting work-around.
The origin of the message that we have dragged or copied into Devonthink from Mail.app is actually embedded in the “URL” metadata. Here is an example of the metadata that is processed and placed there by DT3:
Now, the reply-to variable is actually the message ID. So if you delete everything up reply-to= (including the equal sign) and replace the text with message:,
now when you “Launch” it will open the actual email from Mail.app.
It’s a work-around, but maybe it’s okay for the few where you want to get to the original message.