Your best bet would be to set up the latest issue as the welcome message. To allow for introductions, you might use a "Custom Text" message code to store the current issue and place it in your welcome message. Is this sufficient for your purposes or had you envisioned it another way? I was thinking about allowing a followup #0 to be able to be set up and sent alongside the welcome message, but it's not yet a feature. As for automatically sending the "last sent" message this is a little more difficult and would probably require a new option on the sending screen to differentiate notes to your list from the official newsletter - there's a few things to think about.
Regards