Rejoice! I learned that it is possible to use jQuery's metadata plugin within XSL today. I had previously been stymied because the metadata plugin uses curly brackets: { }.
Trying to use these within an attribute tries to access an XPath node, and that's not the goal here. Thankfully, you can "escape" these curly brackets by simply using two: {{ }}!
<th colspan="2" class="{{sorter: false}}" />
I'm using the metadata.js plugin to set features of tablesorter.
New XSL related link: http://sourceforge.net/projects/xslet/