I finally tried using var_dump() on $item and $_product, which are both used in the email-order-items.php template. $_product revealed a post object, which itself has a post_excerpt property, which looks like it holds the contents of the “Product Short Description” from the WooCommerce product form.
So, to add the description beneath the item name, I added this to my code:
echo '<br/>' . $_product->post->post_excerpt;
Voilà! The short description appears in the email!
Hope this helps someone else! Took me a couple hours of frustration to get it.