I set up the new dnnx xsl extension
add this: xmlns:dnnx="http://www.dotnetnuke.com/xml" extension-element-prefixes ="dnnx"
to my style sheet directive

and these in the first template.

<xsl:apply-templates select="dnnx:setPageTitle($PFB_Title + $PFB_date)" /> <!-- for testing xml token extension --> <xsl:apply-templates select ="dnnx:setPageDescription('test of description token' + $PFB_Title + $PFB_date)" /> <xsl:apply-templates select ="dnnx:registerStyleSheet('/portals/0/myimportant.css')" /> 

and it gives this output in the generated page source.
< title>

NaN

</title>
< meta id="MetaDescription" name="DESCRIPTION" content="NaN" />

css did work:
< link href="/portals/0/my-important.css?cdv=23" type="text/css" rel="stylesheet"/>

I tried without the variables for the title text

<xsl:apply-templates select="dnnx:setPageTitle('test title')" /> <!-- for testing xml token extension --> 

and that seems to work:
< title>

test title

</title>

so do variables need to be used in some other way? those variables are used on the page content and work just fine.

my test page:
http://wga.wbtdev.net/en/test-xml