Yes, it uses XBL (xml binding language) a nice feature of low level FF, closely related to XUL.
I agree that it could be done easily by the user, and adding it to the wiki is a good idea. But there should be a comment as well on the datatable page of the tomahawk components to this wiki entry. But if it could be integrated in the datatable it would be even better, and for someone who knows how to build jsf components not to much work:
- add the js file
- add the css entry
- add a class 'overflow' to the table tag
- add a <div class="underflow">.......</div> in each td for the column where the user specified the attribute ellipses="true"