Do callouts on images work?

I've got a bunch of screenshots containing hand-drawn arrows and annotations that I think would be better done as callouts, but I don't seem to be able to get callouts rendering properly to PDF.

Do they actually work?

If they do, does anyone have a working example they could share? Here's my non-working XML:

    <mediaobjectco>
      <imageobjectco>
        <areaspec units="calspair">
          <area coords="5000,5000 5100,5100" id="p1" linkends="c1"/>
        </areaspec>

        <imageobject>
          <imagedata fileref="/path/to/screenshot.png" scale="50"/>
        </imageobject>

        <calloutlist>
          <callout arearefs="p1">
            <para>Here I am in the middle.</para>
          </callout>
        </calloutlist>
      </imageobjectco>
    </mediaobjectco>

[I'm not sure what purpose the linkends="c1" serves, if any.]

With the default docbook.xsl stylesheet and XEP this renders as a completely plain screenshot without any overlaid numbers. The callout underneath is rendered OK as a white 1 in a black circle.

Chris