I was about to start writing this one out myself, but you posted this right before I had to, which I'm glad for because I didn't realize before I started how big of a hack this would be. Saved me alot of time.
One thing to note is that the font in the "HTML/Plain Text Emails" option on the Edit Options page is not the same as the rest of the page. This is easily fixed by modifying the modifyoptions, registeradult and registercoppa templates, and replacing
PHP Code:
<b>HTML/Plain Text Emails?</b>
with
PHP Code:
<normalfont><b>HTML/Plain Text Emails?</b></normalfont>
Something to point out for those who haven't tried it yet, you can personalize the email in the html email templates by including the same php code that is located in the non-html email templates.
Great hack, Logician.