I'm not sure how widely supported
background images are, but you could give a span the background image??
Code:
span.under {
background-image: url("../Images/under.gif");
background-repeat: no-repeat;
background-position:center bottom
}
<p>...in a rapturous frenzy, <span class="under">that she read to write.</span>
So there is an irony there....</p>
You will want to make sure it's a .png or .gif with transparency, and you'll have to make sure the image is tall enough so that it vertically aligns properly.
You could also add css that would provide for those systems that don't render background images (eg text-decoration: underline), but someone else with more experience in that area would have to help with that.
Hope that helps!