How to get textobject as an alt attribute

I can not get the textobject inside the mediaobject to appear as an
alt attribute in the html file after converting from sgml. Look at this
sgml code:

-----
    <mediaobject>
      <imageobject>
	<imagedata  Fileref="./logo.png" align="center">
      </imageobject>
      <textobject>
	<phrase>Logo</phrase>
      </textobject>
    </mediaobject>
-----

I'm using dsssl stylesheets 1.78. What code could I put into my
customized dsl file to get this working?

Lars