I could not find any nice way to make the "Index" title of the index span the whole page width even if the index is in two columns.
I had to override the whole template.
searching on mailing list archives etc I found the component.titlepage.properties attribute set which is supposed to be used for that (by specifying the span="all" attribute).
However at the moment index.xsl doesn't use component.titlepage.properties
proposed patch
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I patched my local copy in the following way and it seems to work fine.
I can specify
<xsl:attribute-set name="component.titlepage.properties">
<xsl:attribute name="span">all</xsl:attribute>
</xsl:attribute-set>
in my cutomization file
File Added: indexspan.patch
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Patch has been applied in svn