I have been using zmmboxsearch for mail that I have synced to pull out particular messages. You could use this same logic to restore one piece of mail. So restore your mailbox to a new-prefixed username. Then do a search for the message on the restored account, and do an add message to the end-user's account.
After you restore to restored_user, get the piece of mail you are looking for
That will dump anything that matches your query to /tmp with a format of sequence_accountid_messageid.
zmmboxsearch -m email@example.com -q "query string" -d /tmp/
You can then add that message to the original user by doing:
zmmailbox -z -m firstname.lastname@example.org am /tmp/sequence_accountid_messageid