I had also posted this question at the Tech Community, and received this reply from Steven Collier. It makes clear that this requires actual development of an app that runs in Teams meetings.
There is no extensibility on the pre-join screen, so you won't be able to do it there.
You could develop a Teams meeting app that could send a message as new meetings start, it would require developer skill to build the app and plan it's deployment. It would be true development, far more than configuration. You can start reading about meeting apps at Apps for Teams meetings - Teams | Microsoft Docs.