SetField [ YoutTable::YourTextField ; List ( YourTable::YourTextField ; Char ( 9 ) & "● " & "bulleted text here" ) ]
will append a tab, a bullet and some text to any text already in the field, starting it on the next line below the last line of text in your field. You can set a tab stop for the tab on the Inspector's appearance tab.
Thank you Phil! Well, it worked correctly for the most part, except I'm not sure which script trigger to choose. I tried all of the ones they allow, but none of them work exactly the way I'd like. Generally, they all insert the bullet immediately, but none allow me to press enter after the first bullet, have another bullet come up, or indent the bullet in to make a second level bullet.
In my solution--an iPhone based solution where I used to build a multi-line outline style block of text used for formatted input, I clicked a button to run the script and add a bullet item as needed. In my case, I didn't use the script to add the text after the bullet, I just clicked a button to add the tab and bullet, then entered the rest manually.