You could try h3 with a left margin in % or pts so it remains invariant with font size and then use a very slightly larger left margin (also in % or pts) for a separate img carrying the underline gif. Varying the line-height on the img should give you enough control on its vertical position.
e.g.
<h3 class="chaphead">...</h3>
<div class="ornament"><img src="underline.gif" alt="" class="underlineimg"</img></div>
h3 {
margin-left: 35%; <-- alter to suit
...
}
.ornament {
margin-top: 0;
margin-left: 37%; <--alter to suit
vertical-align: top;
line-height: 0.5; <--alter to adjust vertical spacing
}
It's fiddly though, and This sort of fine-grained positional control not really supported by the css subset used in the epub standard. Might be easier just to render your chapter headings as images and embed those.