Note: this needs to be added tot he functions.php file, not the plugin js file. In case anyone else wasn't sure, I figured I'd note that.
The site I wanted to customize with formatted tag descriptions is using the free Mystique theme. That theme includes an Advanced menu field for adding bits of code to the functions.php file without doing a manual edit, which made this very easy to do. It will also be easier to keep track of if I upgrade WordPress - otherwise I might accidentally overwrite the functions.php file and remove this customization.
Of course you also need to have the descriptions enabled in your archives.php page already as well, which in my case was added as this line:
<?php elseif(is_tag()): ?><br />
<div id="tagDescription"><br />
<h2 class="title archive-tag"><?php printf( __('Posts tagged %s', 'mystique'), '<span class="altText">'.single_cat_title('', false).'</span>'); ?></h2><br />
<p><?php echo category_description($category); ?></p><br />
I've added the wrapping div to allow specially styled description that's added to my style.css - that way my header and paragraph look the way I want without affecting the rest of the site.