Thanks for the reply.  Looks like what I need, but using FOP and not
supported (alas).

There's the good old spacer method:
 <fo:table ... >
   <fo:table-column column-width="0.1pt"/> <!--spacer column-->
   ...
   <fo:table-body>
     <fo:table-row>
       <fo:table-cell>
         <fo:block>
            <fo:external-graphics src="spacer.gif"
               heigth="1cm" width="0.05pt"/>
         ...