<a>
  <xsl:attribute 
  name="href"><xsl:text>&start-comment;</xsl:text>fff<xsl:text>&end-comment;</xsl:text></xsl:attribute>
  </a>
  This doesn't work. 

It should work I think, although I'd write it as

 <a href="&start-comment;fff&end-comment;"/>

Did you specify the character map in xsl:output?

David