Scott Grayban mentioned phpLedMailer in the forums here:
http://www.virtualmin.com/forums/general-discussion/question-about-phpli...
It looks like a good simple alternative to phpList, which is currently the only way to send out HTML newsletters with Virtualmin. phpList works great, but it is very complex for new users to set up and get used to. phpLedMailer, on the other hand, appears to be user friendly. You can find more information and download it here:
http://www.ledscripts.com/free/php/phpledmailer
One thing to keep in mind with this script is that it must be installed multiple times if a user wants to use it with more than one mailing list. Therefore, the install script must take this into account and request the name of the list as part of the install, so that additional installs will not step on each other. I would assume that each one needs to be installed in its own directory for the script files and its own database instance.