Here we can collect strategies and next steps ....
(A) As a short answer to this complicated question I will install a dummy user "mailinglist" who will sent text mentioning him to the mailinglist(s?).
We discussed also other strategies:
(B) When creating a new thread in discourse, write a short summary to the mailinglist, linking to the topic
We could combine (A)+(B) if we start new threads with a short introduction into the thread and add the main content as a first reply. So Mailing list users will get an info about the new thread, but will have to come to discourse, to read full information.