Do mean like content in a block??? To do this you need to make sure your permissions are set so anonymous users can see the page. Go to the blocks area (Admin:Blaocks:List all blocks), click on edit and make sure the content is visible to all groups.
I just checked and I cannot find a place to edit allowed extensions either. My guess is if someone is to change it, it must be done in the code somewhere.
Good, I just finished an hour ago using a mass mailer to send out my first newsletter, so the timing is perfect. I hope it will allow for html mail. Regardless, I cannot wait to see the module!