defining output document type by stylesheet parameters

Hello all,

In XSLT 2.0, is it possible to define the output document type by
stylesheet parameters?  I've tried this without success by the following
method:

<xsl:stylesheet version="2.0"
xmlns:xsl="http://www.w3.org/1999/XSL/Transform">
   <xsl:param name="doctype.system" select="string('commbook.dtd')"/>
   <xsl:param name="doctype.public"
      select="string('-//JMP//DTD DocBook V4.4-Based Subset
V0.1//EN')"/>
   <xsl:output method="xml" version="1.0" encoding="utf-8"
doctype-system="{$doctype.system}"
      doctype-public="{$doctype.public}" />

I expect to get this output:

	<!DOCTYPE chapter PUBLIC "-//JMP//DTD DocBook V4.4-Based Subset
V0.1//EN" "commbook.dtd">

But instead receive this output:

	<!DOCTYPE chapter PUBLIC "{$doctype.public}"
"{$doctype.system}">

I know this type of thing wasn't possible in 1.0, but it seems like it
might be in 2.0.  Is there another approach I should be using, or am I
out of luck?  I'm using Saxon 9B.

Thanks,

-James

-----
James Sulak
Electronic Publishing Developer
Jones McClure Publishing