Keith,
Unless something has changed, Tumblr allows control of the HTML themes that it uses to display content. You should be able to paste the Munchkin code directly into the theme. Here are some instructions to put you on the right path: chrisabraham.com/2011/04/06/how-to-add-custom-html-javascript-etc-code-to-your-tumblr/
Not sure why you want a hosted blog when you could easily do a self-hosted WordPress blog and have full control of your site and hosting, but that's a topic for another thread 🙂