I just realized that TextMate includes a bundled command to preview Markdown-formatted text. Since I'm using the Markdown option for formatting in Typo, that means I can use TextMate as my blogging tool. I just save a file named
post.markdown to get TextMate to notice that it's in Markdown format. Then the key combo CMD-CTRL-OPT-P generates a preview of the formatted output. Copy and paste into Typo's article creation form and I'm done!